Search

Laboratory experiments indicated that Potamogeton nodosus Poir, winter buds and Potamogeton pectinatus L. tubers germinated in response to water-saturated substrate at 15, 20, and 25°C. Outdoor experiments with intact sediment cores indicated that winter buds of Potamogeton gramineus L. also germinated in response to saturating conditions at low temperatures (10–15°C). P. gramineus winter buds and P. pectinatus tubers, collected from irrigation canals during the winters of 1985 and 1986, germinated within 10 days of being placed at 25°C, indicating that germination was inhibited by environmental conditions and not by innate dormancy. Measurements made in the springs of 1985, 1986, 1987, and 1989 in the Byrnes canal, demonstrated that terrestrial forms of P. gramineus were present at densities ranging from 50 to 746 m −2 , accounting for 30–96% of the total number of winter buds present per square meter. Penetration of accumulated rainwater into the sediment appeared to influence germination of P. gramineus winter in buds and may explain why deep winter buds do not germinate prior to the canal filling with water in the spring. The ability to induce formation of terrestrial forms may lead to new management approaches.

Interferon and interferon inducers are well known to depress the cytochrome P450-dependent hepatic mixed-function oxidase system and cause a decrease in the capacity of the liver to metabolize drugs and xenobiotics. In this study we have shown that the interferon-mediated changes in an induced form of hepatic cytochrome P450 (CYP4A) are mediated via a depression in the levels of mRNA as assessed by Northern blot and slot blot analyses using a 20-base synthetic oligodeoxyribonucleotide hybridization probe. Rats were pretreated with clofibrate to maximize CYP4A mRNA levels prior to the administration of polyinosinic acid.polycytidylic acid (poly IC), an alpha/beta interferon inducer. Hepatic CYP4A mRNA levels were decreased by 49 and 30% at 6 and 24 hr, respectively, following poly IC administration. In hepatic microsomes cytochrome P450 and functional CYP4A as measured by lauric acid hydroxylation, were not affected at 6 hr, but were depressed by 39 and 27%, respectively, 24 hr following poly IC administration. These results suggest that interferon depresses induced levels of hepatic drug metabolism by lowering the level of cytochrome P450 mRNAs and subsequent synthesis of cytochrome P450 apoproteins.

Hydrilla verticillata (L.f.) Royle produces two types of vegetative propagules, subterranean turions and axillary turions. After 8 or 12 weeks growth under similar conditions, plants grown from subterranean turions weighed 1.7 to 2 times as much as plants grown from axillary turions. Subterranean turion-derived plants produced more propagules (by weight and number) per plant than plants from axillary turions. Characteristics of weight frequency distributions (median, minimum, maximum, and coefficient of variation) for new subterranean turions were also influenced by the type of propagule from which the parent plant was derived. The number of root crowns per plant which reflects the plant's ability to expand horizontally was significantly greater for subterranean turion-derived plants (by 2 times) than for plants from axillary turions. These results support the hypothesis that following colonization of an area the impact ofHydrilla on resident species changes over time.

Early growth of Potamoaeton gramineus L. was evaluated at several combinations of temperature (10 to 35 C) and light (7 to 213 umol m−2 s−1) in laboratory cultures. Leaf production rates were positive except at 35 C, and generally increased at higher light levels. Leaf production was reduced at 10 or 35 C. At low light levels ( 65 umol m−2 s−1) dry weight increased and appeared to be a function of temperature. At ≥ 65 umol m−2 s−1 P. gramineus was photosynthetically sufficient within 4 weeks of sprouting. Comparison of P. gramineus and P. pectinatus showed that P. gramineus would not grow as rapidly under early season conditions.

A restriction map of the 35-kb circular DNA molecule of Plasmodium falciparum showed that a region of about 6 kb, encoding both a large and a small subunit ribosomal RNA gene, has been duplicated in inverted orientation. The complete sequence of one small subunit rRNA gene is presented as well as an analysis of transcripts from erythrocytic stage parasites. Comparative sequence analysis of the rRNA gene and the proposed secondary structure of the rRNA suggest that it is of organellar origin. Intriguingly, while some characteristics of the small subunit rRNA gene are similar to mitochondrial sequences, others are more like those of plastids. The origin of the circular DNA molecule and evolutionary implications of its genetic content are discussed.

Potamogeton gramineus L., growing in a northern California irrigation canal, was reddish-brown due to the presence of anthocyanin pigments. Tissue N and P were positively correlated with relative anthocyanin content. This suggests that anthocyanins did not accumulate due to N or P limitation. The anthocyanin content of P. gramineus grown in Byrnes Canal sediment, in the canal or in a greenhouse, was also unaffected by the addition of N, P, or N and P. Inorganic N concentrations in sediment samples from the Byrnes Canal were > 11 mg kg−1, which was the level of inorganic N associated with increased anthocyanin production in a greenhouse study. The addition of Fe to Byrnes Canal sediments did not influence the relative anthocyanin content of P. gramineus grown in the canal. P. gramineus grown at 25 combinations of temperature (10, 15, 25, 30 and 35°C) and light (8, 15, 22, 58 and 189 μmol m−2 s−1) in laboratory cultures exhibited greater changes in relative chlorophyll content than in relative anthocyanin content. In general, chlorophyll decreased sharply with increasing light while anthocyanin increased slightly. In the context of the light and temperature conditions prevalent in the Byrnes Canal, the reddish-brown color of P. gramineus appears to be the consequence of a decrease in relative chlorophyll content which unmasks the anthocyanins.

Results from experiments with Potamogeton pectinatus L. grown in an artificial substrate amended with various combinations of peat and glucose or sucrose suggest that plant weight and the number and weight of tubers produced per plant increased significantly when plants were grown in sediment amended with peat. Contrary to expectations, the addition of labile organic components, glucose or sucrose, to the substrate (up to 15%, w/w) did not influence plant growth. Plants grown in sediment amended with peat had higher levels of tissue Fe. The weight of tubers produced per gram of non tuber tissue was not influenced by sediment properties. These results support the hypothesis that the mechanism by which sediment organic matter affects macrophyte growth is by altering sediment density.

The weight of Potamogeton gramineus L. winter buds was greatest between 6 and 10 cm deep in the sediment for samples collected from two northern California irrigation canals. The relationship between winter bud weight and depth was adequately described by a polynomial equation containing a quadratic term, but coefficients in equations were different for two canals. For buds planted at various depths, after 10 weeks growth measurements in terms of plant dry weight, shoot length, rhizome length, number of flowers, ramets, leaves, and floating leaves per plant were all negatively related to planting depth. Plants growing from greater depths had lower rates of shoot elongation, leaf production, and ramet production. Allocation of biomass to photosynthetic tissues and flowers was also influenced by initial planting depth, as was the time of emergence from the sediment. These findings suggest that predicting macrophyte growth and phenology from environmental changes alone may not be adequate.

To date, direct analysis of mitochondrial proteomes has largely been limited to animals, fungi and plants. To broaden our knowledge of mitochondrial structure and function, and to provide additional insight into the evolution of this key eukaryotic organelle, we have undertaken the first comprehensive analysis of the mitochondrial proteome of a protist. Highly purified mitochondria from Tetrahymena thermophila, a ciliated protozoon, were digested exhaustively with trypsin and the resulting peptides subjected to tandem liquid chromatography-tandem mass spectrometry (LC/LC-MS/MS). In this way, we directly identified a total of 573 mitochondrial proteins, 545 of which are encoded by the nuclear genome and 28 by the mitochondrial genome. The latter number includes a novel, 44 residue protein (which we designate Ymf78) that had not been recognized during annotation of the T. thermophila mtDNA sequence. The corresponding gene, ymf78, is highly conserved in genomic position, size and sequence within the genus Tetrahymena. Our analysis has provided broad coverage of both membrane-bound and soluble proteins from the various submitochondrial compartments, with prominent representatives including components of the tricarboxylic acid cycle, Complexes I-IV of the electron transport chain and Complex V (ATP synthase), the mitochondrial transcription and translation machinery, the TOM and TIM protein translocases, various mitochondrial transporters, chaperonins (Cpn60, Hsp70, Hsp90), at least four FtsH family ATP-dependent metalloproteases implicated in m-AAA and i-AAA protease function, and enzymes involved in lipid, amino acid and coenzyme metabolism, as well as iron-sulfur cluster formation. Unexpectedly, six of the ten enzymes of glycolysis were found by MS analysis of purified T. thermophila mitochondria, whereas no hits were seen to any cytosolic ribosomal proteins. At least one of the glycolytic proteins, enolase, has an evident N-terminal extension that exhibits characteristics of a typical mitochondrial targeting peptide. As in other organisms, phylogenetic analysis of functionally annotated mitochondrial proteins demonstrates that

The minor spliceosome is a ribonucleoprotein complex that catalyses the removal of an atypical class of spliceosomal introns (U12-type) from eukaryotic messenger RNAs. It was first identified and characterized in animals, where it was found to contain several unique RNA constituents that share structural similarity with and seem to be functionally analogous to the small nuclear RNAs (snRNAs) contained in the major spliceosome. Subsequently, minor spliceosomal components and U12-type introns have been found in plants but not in fungi. Unlike that of the major spliceosome, which arose early in the eukaryotic lineage, the evolutionary history of the minor spliceosome is unclear because there is evidence of it in so few organisms. Here we report the identification of homologues of minor-spliceosome-specific proteins and snRNAs, and U12-type introns, in distantly related eukaryotic microbes (protists) and in a fungus (Rhizopus oryzae). Cumulatively, our results indicate that the minor spliceosome had an early origin: several of its characteristic constituents are present in representative organisms from all eukaryotic supergroups for which there is any substantial genome sequence information. In addition, our results reveal marked evolutionary conservation of functionally important sequence elements contained within U12-type introns and snRNAs.

This study explored the potential use of hyperspectral data in the non-destructive assessment of chlorophyll, carbon, and nitrogen content of giant reed at the canopy level. We found that pseudoabsorption and derivatives of original hyperspectral data were able to describe the relationship between spectral data and measured biochemical characteristics. Based on correlogram analyses of ground-based hyperspectral data, we found that derivatives of pseudoabsorption were the best predictors of chlorophyll, carbon, and nitrogen content of giant reed canopies. Within the visible region, spectral data significantly correlated with chlorophyll content at both 461 nm and 693 nm wavelengths. Within the near-infrared region, carbon levels correlated with hyperspectral data at five causal wavelengths: 1038 nm, 1945 nm, 1132 nm, 1525 nm, and 1704 nm. The best spectral wavelength for estimating nitrogen content was 1542 nm. Such relationships between nutrient content and spectral data were best represented by exponential functions in most situations.

Acanthamoeba castellanii is a free-living amoeba found in soil, freshwater, and marine environments and an important predator of bacteria. Acanthamoeba castellanii is also an opportunistic pathogen of clinical interest, responsible for several distinct diseases in humans. In order to provide a genomic platform for the study of this ubiquitous and important protist, we generated a sequence survey of approximately 0.5 x coverage of the genome. The data predict that A. castellanii exhibits a greater biosynthetic capacity than the free-living Dictyostelium discoideum and the parasite Entamoeba histolytica, providing an explanation for the ability of A. castellanii to inhabit a diversity of environments. Alginate lyase may provide access to bacteria within biofilms by breaking down the biofilm matrix, and polyhydroxybutyrate depolymerase may facilitate utilization of the bacterial storage compound polyhydroxybutyrate as a food source. Enzymes for the synthesis and breakdown of cellulose were identified, and they likely participate in encystation and excystation as in D. discoideum. Trehalose-6-phosphate synthase is present, suggesting that trehalose plays a role in stress adaptation. Detection and response to a number of stress conditions is likely accomplished with a large set of signal transduction histidine kinases and a set of putative receptor serine/threonine kinases similar to those found in E. histolytica. Serine, cysteine and metalloproteases were identified, some of which are likely involved in pathogenicity.

A variation of the basic Cockroft-Walton (C-W) Voltage Multiplier circuit design may be used to generate multiple voltages at sufficient currents to drive the dynodes of a photomultiplier tube. In a battery-operated handheld device, the current draw on the batteries must be kept to a minimum. Several other parameters must be considered carefully during the design as well. Components must be chosen based on size restrictions, expected load current, expected output voltage range, and the maximum allowable ripple in the output voltage. A prototype surface mount C-W board was designed and tested to power two photomultipliers. The whole system, including the detectors, draws less than 15 mA of supply current with the outputs at 1000 VDC.

Currently at the INEEL, a handheld device is being developed to measure fast neutrons and gamma rays using a single detector instead of a previous two detector system. The handheld detection system presented here uses a single 1/2 inch (diameter) by 1/2 inch (long) liquid scintillator detector (BC501). This means the detection system can be made smaller, lighter, less expensive, and is expected to be more sensitive than the original system. A smaller and lighter device makes it possible to be used in several applications such as customs inspection, border security, environmental radiation monitoring, and so on. The use of only one detector requires that the neutrons and gamma rays be distinguished by the shape of their pulses in the detector. Two methods of pulse shape discrimination (PSD) are: presented here, charge integration and crossover timing. Figures of merit were calculated for both methods for a threshold energy range of 50 to 600 keV. Results show that the crossover method gives much better PSD for electron energy of 100 keV and lower, whereas the charge integration method leads to better separation above 100 keV. However, the neutrons and gamma rays are totally separated for energies of 100 keV and above in both techniques. We are currently designing a miniaturized electronic system to be incorporated in the handheld device.

Aquatic plants growing in flowing waters frequently inhabit dynamic substrates, subject to erosion or sediment deposition. Rooted macrophytes have disappeared from the upper portions of Fall River, CA where from 0.6 to 1.2 m of sandy sediments have accumulated. We assessed the abundance of Zannichellia palustris L. seeds in the seed bank, and performed experiments to determine the effect of sediment accumulation on germination and emergence of Z. palustris seedlings. In 1996 and 1997, sediment cores were collected from the upper 15 km of Fall River. Cores were placed in a greenhouse and germination monitored for the next 30–40 days. Viable germinating Z. palustris seeds were present in 67% of the cores collected in 1996. Z. palustris seed density was 1219 seeds m −2 on average and ranged from 0 to 5920 seeds m −2 . Seed abundance did not differ between the upper ( 11 cm) of cores collected in 1996 or 1997. We conducted four experiments in which Z. palustris seeds and natural sediments containing Z. palustris seeds were buried at various depths under sand. Burial by more than 2 cm of sand inhibited germination and emergence. The implication is that significant sediment accumulation may disrupt natural annual regeneration of Z. palustris from seeds, although a viable seed bank is maintained for several years.

Despite its large size (200-2400 kilobase pairs), the mitochondrial genome of angiosperms does not encode the minimal set of tRNAs required to support mitochondrial protein synthesis. Here we report the identification of cytosolic-like tRNAs in wheat mitochondria using a method involving quantitative hybridization to distinguish among three tRNA classes: (i) those encoded by mitochondrial DNA (mtDNA) and localized in mitochondria, (ii) those encoded by nuclear DNA and located in the cytosol, and (iii) those encoded by nuclear DNA and found in both the cytosol and mitochondria. The latter class comprises tRNA species that are considered to be imported into mitochondria to compensate for the deficiency of mtDNA-encoded tRNAs. In a comprehensive survey of the wheat mitochondrial tRNA population, we identified 14 such imported tRNAs, the structural characterization of which is presented here. These imported tRNAs complement 16 mtDNA-encoded tRNAs, for a total of at least 30 distinct tRNA species in wheat mitochondria. Considering differences in the set of mtDNA-encoded and imported tRNAs in the mitochondria of various land plants, the import system must be able to adapt relatively rapidly over evolutionary time with regard to the particular cytosolic-like tRNAs that are brought into mitochondria.

In Crithidia fasciculata, the ribosomal RNA (rRNA) gene repeats range in size from approximately 11 to 12 kb. This length heterogeneity is localized to a region of the intergenic spacer (IGS) that contains tandemly repeated copies of a 19mer sequence. The IGS also contains four copies of an approximately 55 nt repeat that has an internal inverted repeat and is also present in the IGS of Leishmania species. We have mapped the C.fasciculata transcription initiation site as well as two other reverse transcriptase stop sites that may be analogous to the A0 and A' pre-rRNA processing sites within the 5' external transcribed spacer (ETS) of other eukaryotes. Features that could influence processing at these sites include two stretches of conserved primary sequence and three secondary structure elements present in the 5' ETS. We also characterized the C.fasciculata U3 snoRNA, which has the potential for base-pairing with pre-rRNA sequences. Finally, we demonstrate that biosynthesis of large subunit rRNA in both C. fasciculata and Trypanosoma brucei involves 3'-terminal addition of three A residues that are not present in the corresponding DNA sequences.

The fragmented mitochondrial ribosomal RNAs (rRNAs) of the green algae Chlamydomonas eugametos and Chlamydomonas reinhardtii are discontinuously encoded in subgenic modules that are scrambled in order and interspersed with protein coding and tRNA genes. The mitochondrial rRNA genes of these two algae differ, however, in both the distribution and organization of rRNA coding information within their respective genomes. The objectives of this study were (1) to examine the phylogenetic relationships between the mitochondrial rRNA gene sequences of C. eugametos and C. reinhardtii and those of the conventional mitochondrial rRNA genes of the green alga, Prototheca wickerhamii, and land plants and (2) to attempt to deduce the evolutionary pathways that gave rise to the unusual mitochondrial rRNA gene structures in the genus Chlamydomonas. Although phylogenetic analysis revealed an affiliation between the mitochondrial rRNA gene sequences of the two Chlamydomonas taxa to the exclusion of all other mitochondrial rRNA gene sequences tested, no specific affiliation was noted between the Chlamydomonas sequences and P. wickerhamii or land plants. Calculations of the minimal number of transpositions required to convert hypothetical ancestral rRNA gene organizations to the arrangements observed for C. eugametos and C. reinhardtii mitochondrial rRNA genes, as well as a limited survey of the size of mitochondrial rRNAs in other members of the genus, lead us to propose that the last common ancestor of Chlamydomonas algae contained fragmented mitochondrial rRNA genes that were nearly co-linear with conventional rRNA genes.

A tendency for exons to correspond to discrete units of protein structure in protein-coding genes of ancient origin would provide clear evidence in favor of the exon theory of genes, which proposes that split genes arose not by insertion of introns into unsplit genes, but from combinations of primordial mini-genes (exons) separated by spacers (introns). Although putative examples of such correspondence have strongly influenced previous debate on the origin of introns, a general correspondence has not been rigorously proved. Objective methods for detecting correspondences were developed and applied to four examples that have been cited previously as evidence of the exon theory of genes. No significant correspondence between exons and units of protein structure was detected, suggesting that the putative correspondence does not exist and that the exon theory of genes is untenable.

The mitochondrial genomes of wheat and rye each contain a three-member family of recombining repeat sequences (the "18S/5S repeat") that encode genes for 18S and 5S rRNAs (rrn18 and rrn5) and tRNA(fMet) (trnfM). Here we present, for wheat and rye, the sequence and boundaries of the "common sequence unit" (CSU) that is shared between all three repeat copies in each species. The wheat CSU is 4,429 base-pairs long and contains (in addition to trnfM, rrn18 and rrn5) a putative promoter, three tRNA-like elements ("t-elements"), and part of a pseudogene ("psi atpAc") that is homologous to chloroplast atpA, which encodes the alpha subunit of chloroplast F1 ATPase. The rye CSU is somewhat smaller (2,855 base pairs) but contains much the same genic and other sequence elements as its wheat counterpart, except that two of the three t-elements as well as psi atpAc are found in only one of the three downstream flanks of the 18S/5S repeat, outside the CSU boundaries. In interpreting the sequence data in terms of the evolutionary history of the 18S/5S-repeat family of wheat and rye, we conclude that: (1) the wheat-rye form of the 18S/5S repeat most likely originated between 3 and 14 million years ago, in a lineage that gave rise to wheat and rye but not to barley, oats, rice or maize; (2) the close linkage (1-bp apart) between trnfM and rrn18 is similarly limited in its taxonomic distribution to the wheat/rye lineage; (3) the trnfM-rrn18 pair arose via a single mutation that inserted a sequence block containing trnfM immediately upstream of rrn18; and (4) the presence of a putative promoter upstream of rrn18 in all wheat and rye repeats is consistent with all three repeat copies being transcriptionally active. We discuss these conclusions in the light of the possible functional significance of recombining-repeats in plant mitochondrial genomes.

We report the sequence of a 7.2 kilobase pair DNA fragment containing a copy of the wheat mitochondrial gene (rrn26) that encodes the mitochondrial large-subunit ribosomal RNA (26S rRNA). The mature 26S rRNA was determined by direct RNA sequencing to be 3467 nucleotides long, and to share a 5′-terminal pentanucleotide (5′-AUCAU), thought to be important in post-transcriptional processing, with the wheat mitochondrial small-subunit (18S) rRNA. Two other prominent features of the sequence were noted. First, upstream of rrn26 are located two tandem copies of a 70 base pair element containing a putative mitochondrial promoter motif (TCGTATAAAAA). Second, downstream of rrn26 is a sequence element that, if transcribed, would produce and RNA with a secondary structure resembling that of tRNAs but differing sufficiently from the latter structure to preclude any transcript from functioning normally in translation. These upstream and downstream sequence elements may play a role in the expression of rrn26 in wheat mitochondria.

Interest in plant mitochondrial (mt) DNA and RNA has increased dramatically in the last 10 years. The physical complexity of flowering plant mtDNA, first described by Quetier and Vedel (1977), is still a topic of study and is as yet not fully understood. The presence in plant mitochondria of a unique 5 S ribosomal RNA (rRNA) (absent from all other mitochondria) was recognized early and was the topic of several publications from our group (Bonen and Gray 1980; Spencer et al. 1981).

ALTHOUGH it is widely accepted that the plastids of plants and algae originated as endosymbionts1, the details of this evolutionary process are unclear2'3. It has been proposed that in organisms whose plastids are surrounded by more than two membranes, the endosymbiont was a eukaryotic alga rather than a photosynthetic prokaryote4. The DNA-containing5 nucleomorph6 of cryptomonad algae appears to be the vestigial nucleus of such an algal endosymbiont7. Eukaryotic-type ribosomal RNA sequences have been localized to a nucleolus-like structure in the nucleomorph8. In support of the hypothesis that cryptomonads are evolutionary chimaeras of two distinct eukaryotic cells, we show here that Cryptomonas Φ contains two phylogenetically separate, nuclear-type small-subunit rRNA genes, both of which are transcriptionally active. We incorporate our rRNA sequence data into phylogenetic trees, from which we infer the evolutionary ancestry of the host and symbiont components of Cryptomonas Φ. Such trees do not support the thesis3 that chromophyte algae evolved directly from a cryp-tomonad-like ancestor.

An initiator methionine transfer RNA (tRNAfMet) gene has been identified by sequence analysis of a cloned SalI restriction fragment of wheat mitochondrial DNA. The 3′-end of this gene, which does not encode the CCA terminus of the mature tRNA, is separated by only one basepair from the 5′-end of the 18 S ribosomal RNA gene, suggesting that the two genes are part of the same transcriptional unit. The wheat mitochondrial tRNAfMet sequence, the first from plant mitochondria, displays strong structural affinity with eubacterial/chloroplast tRNAfMet sequences.

The effect of nickel on the growth of the freshwater diatom Navicula pelliculosa was determined by noting differences in the growth rate and number of cells present in laboratory cultures exposed to 0, 100, 300 and 600 μg/litre nickel for 14 days. Nickel concentrations as low as 100 μg/litre (1·7 × 10 −6 M) reduced the population growth rate by 50% and led to decreased 14-day cell numbers. Calculation of the equilibrium composition of the culture medium showed that 99·8% of the total nickel added existed as Ni +2 . The occurrence of Ni +2 in the medium was associated with reduced algal growth.

In Escherichia coli B/r the expression of UV inducible (SOS) functions is under the control of the recA and lexA genes. In this study we have characterized mutants which are altered in their ability to express SOS functions. These mutants were isolated as UV resistant UV nonmutable (Rnm) derivatives of the lexA102 uvrA155 mutant strain WP51. The UV resistance of these Rnm strains is a result of the suppression of lexA102 mediated UV sensitivity. Genetic mapping of rnm mutations shows that the two predominant classes, rnmA and rnmB, map in or very near the lexA and recA genes respectively. rnmA mutations differ from rnmB with regard to recA protein synthesis, rnmA mutations do not restore the ability to express high levels of recA protein after UV treatment whereas rnmB mutations result in constitutive expression of high levels of recA protein. However, both rnmA and rnmB mutant strains inhibit postirradiation DNA degradation. This shows that in rnmA strains, high levels of recA protein are not needed to inhibit postirradiation DNA degradation. The genetic map location and constitutive expression of recA protein synthesis resulting from rnmB mutations suggests that they are operator constitutive mutations of the recA gene. The result that the lexA+ gene is required for the expression of UV mutagenesis in rnmB mutants shows that high levels of recA protein do not circumvent the need for the lexA+ gene product in this process. Thus, while the lexA gene product is required for the induction of recA protein synthesis, lexA must have an additional role in UV induced mutagenesis.

Mean summer abundance (no/sq m or mg/sq m) of Keratella cochlearis did not differ significantly at three locations which differed in depth within Eagle Creek Reservoir, Indiana. The relationship between mean summer biomass and depth was thus different from that which has been reported in the literature for Cladocera and copepods. Mean individual wet weights were greater (P< 0.001) at the limnetic location (29.5 μg/individual) than at the 2 shallower sites (27.0, 25.9 μg/individual). The width:length ratio was 0.63 for populations at all three sample sites.

When recently germinated Potamogeton pectinatus L. tubers were grown at temperatures ranging from 10 to 37°C under low irradiances (6–120 μE m −1 s −1 ), temperature exerted a stronger influence on levels of the pigments chlorophyll a and chlorophyll b than did irradiance. Temperature and irradiance interacted to influence the level of total carotenoids. At low irradiances, changes in temperature modified the concentrations of accessory pigments relative to chlorophyll a . Two indicators of growth were also strongly affected by temperature. The relative rate of shoot elongation varied from −0.04 to 1.23 mm cm −1 day −1 . At 23–30°C, the relative shoot elongation rate was inversely related to irradiance, but at temperatures outside this range irradiance had no influence on the relative shoot elongation rate. The relative leaf production rate varied from 0 to 0.28 leaves leaf −1 day −1 and increased with temperature and irradiance. No leaves were produced at 37°C. These results indicate that the optimum temperature for early growth of P. pectinatus is near 23–30°C, and that, during the early growth period, temperature may be as important as irradiance in influencing growth of P. pectinatus .

We have begun a systematic search for potential tRNA genes in wheat mtDNA, and present here the sequences of regions of the wheat mitochondrial genome that encode genes for tRNA(Asp) (anticodon GUC), tRNA(Pro) (UGG), tRNA(Tyr) (GUA), and two tRNAs(Ser) (UGA and GCU). These genes are all solitary, not immediately adjacent to other tRNA or known protein coding genes. Each of the encoded tRNAs can assume a secondary structure that conforms to the standard cloverleaf model, and that displays none of the structural aberrations peculiar to some of the corresponding mitochondrial tRNAs from other eukaryotes. The wheat mitochondrial tRNA sequences are, on average, substantially more similar to their eubacterial and chloroplast counterparts than to their homologues in fungal and animal mitochondria. However, an analysis of regions ∼ 150 nucleotides upstream and ∼ 100 nucleotides downstream of the tRNA coding regions has revealed no obvious conserved sequences that resemble the promoter and terminator motifs that regulate the expression of eubacterial and some chloroplast tRNA genes. When restriction digests of wheat mtDNA are probed with (32)P-labelled wheat mitochondrial tRNAs,20 hybridizing bands are detected, whether enzymes with 4 bp or 6 bp recognition sites are used. This suggests that the wheat mitochondrial genome, despite its large size, may carry a relatively small number of tRNA genes.

The zooplankton community in Lake Charles East, Indiana, was sampled from June, 1974 through September, 1977 as part of a lake restoration study. About 1.8 × 104 kg of lime and 1.8 × 106 kg of ponded fly ash were added to the lake during May through August, 1975 to precipitate phosphate and seal the sediments. Annual mean species number (5.2–11.3) and annual mean species diversity (H′, 0.9–1.3) were highest in 1976, the first year after treatment. By the second post-treatment year these variables had returned to pretreatment levels. Prior to treatment Cladocera were dominant during fall and early winter (Sept.–Dec. 1974), with Copepoda dominant in late winter and spring (Jan.–May 1975). After treatment Cladocera were dominant throughout the fall and winter (Nov. 1957–May 1976). Copepoda were again dominant in August 1976. The short term effect of the treatment appeared to be termination of the latter part of the copepod annual cycle through elimination of aestivating copepodites in the summer and increased abundance of Cladocera during the winter and spring immediately following treatment. Community composition one year after treatment was similar to that observed prior to treatment.

The effect of nickel on the growth of seven species of freshwater algae was assessed in a chemically defined medium. Pediastrum tetras, Ankistrodesmus falcatus, A. falcatus var. acicularis, Scenedesmus quadricauda and S. dimorphuu exhibited decreased growth at 1·7 μmol litre −1 Ni. Anabaena flos-aquae was unaffected up to 10·2 μmol litre −1 Ni and A. cylindrica was not affected by 10·2 μmol litre −1 Ni. Calculation of the theoretical chemical equilibrium indicated that 98% of the added nickel existed as the free nickel ion, Ni +2 . Measurements of phytoplankton productivity in a shallow lake indicated that nickel (1·7, 5·1 and 10·2 μmol litre −1 ) did not inhibit community photosynthesis on two dates when the plankton community was dominated by species of blue-green algae.

In the course of isolating tRNA genes from wheat mtDNA, we have found the same tRNA(Pro) gene in two different Hind III restriction fragments, H-P1 (0.7 kbp) and H-P2 (1.7 kbp). Sequences immediately flanking these duplicate genes are closely related, although not identical; sequence comparisons suggest that multiple rearrangements have occurred in the vicinity of the H-P2 tRNA(Pro) gene, relative to the H-P1 version. The chimeric nature of H-P2 is emphasized by the presence of sequences that are also found upstream of the wheat mitochondrial 26S rRNA gene, as well as sequences derived from chloroplast DNA. Comparison of H-P2 with H-P1 plus upstream sequences provides some insight into possible molecular events that might have generated H-P2. In particular, such comparisons suggest a model in which the homologous sequences in H-P2 are seen to be derived from H-P1 plus upstream sequences as a result of an intragenomic, site-specific rearrangement event, followed by amplification of the product, its fixation in the mitochondrial genome, and subsequent sequence divergence (single base changes as well as insertions/deletions of up to 50 nucleotides). The results reported here implicate particular primary sequence motifs in certain of the rearrangements that characterize H-P2.

In greenhouse experiments, sago pondweed plants were grown from three different size classes of tubers and treated with four levels of fluridone. Results indicate that although fluridone significantly reduced mean plant weight, the extent of the reduction in weight depended on the size of tuber from which the plant was grown. Mean plant weight was greater for plants exposed to fluridone (up to 0.5 mg/L) for 7 days, if the plants were initially from larger tubers (51 to 60 or 101 to 200 vs. 11 to 20 mg fresh weight). Mean plant length and the number of ramets/plant were affected in a similar manner. The results underscore the importance of specifying the size of tubers used in growth studies and contribute to understanding of the roles that biological factors play in regulating aquatic plant response to herbicide treatment.

A la dose herbicide recommandee (1 ppm) l'aquashade ne modifie pas sensiblement la consommation d'oxygene de l'ecrevisse

Rates of net photosynthesis and respiration were determined for Pithophora oedogonia (Mont.) Wittr. acclimatized to 56 combinations of light (7–1200 μE m −2 s −1 ) and temperature (5–35°C). Conditions for maximum net photosynthesis were estimated to be 26°C and 970 μE m −2 s −1 . The rate of net photosyntheses varied considerably with temperature, with the maximum measured value (9.67 mg O 2 h −1 g dry wt. −1 ) occurring at 25°C. Respiration rate increased with temperature and the light received just prior to measurement. The maximum respiration rate (7.05 mg O 2 g −1 h −1 ) occurred at 30°C and 1200 μE m −2 s −1 . Exposure of Pithophora to light levels of 600 or 1200 μE m −2 s −1 prior to determination of the respiration rate resulted in significantly elevated levels of oxygen consumption at temperatures ≥ 15°C. The relationship between light, temperature and photosynthesis and respiration were summarized as three-dimensional response surfaces.

We compared three methods for determining the chlorophyll a content of several aquatic macrophytes. Extraction with DMSO (dimethyl sulfoxide; 100%) yielded chlorophyll a values which were equivalent to or slightly greater than the values for samples extracted with 90% acetone or 90% methanol. Chlorophyll a extraction with DMSO was somewhat more efficient than methanol or acetone depending on the species examined. Chlorophyll a in DMSO extracts appeared to be more stable over time. The DMSO procedure is relatively quick and does not require maceration or centrifugation of the sample, and appears to be suitable for a variety of submersed aquatic macrophytes.

Two species of green algae, Scenedesmus quadricauda (UTEX 76) and Ankistrodesmus falcatus var. acicularis (UTEX 101), were grown in a chemically defined medium containing 10·2 μm Ni. The concentration of free metal ion was manipulated by adding the chelators EDTA (ethylenediamine tetraacetate) or NTA (nitrilotriacetic acid). For both algal species, growth rates were inversely related to the calculated concentration of free divalent nickel and independent of the total nickel in solution. For A. falcatus var. acicularis, the 14-day cell number was inversely related to the free divalent nickel concentration, while the mean individual cell volume decreased as the free divalent nickel concentration lessened. Nickel inhibition of algal growth appears to be related to the presence of the free divalent ion and is thus similar to copper, zinc and cadmium.

Thirty-eight percent of monoecious hydrilla [Hydrilla verticillata (L.f.) Royle #3 HYLLIJ grown from tubers produced new tubers after 28 days exposure to a 10-h photoperiod. One hundred percent of the plants grown at a 10-h photoperiod produced tubers by 56 days while only thirty-eight percent of those grown at a 12-h photoperiod did so. Plants grown at 14or 16-h photoperiods did not produce tubers. Tubers appeared to be produced at the expense of new root and shoot tissue. Dioecious hydrilla (female) grown under similar conditions did not produce tubers by 56 days at any photoperiod examined. Relative growth rates (total dry weight) for both types did not differ with photoperiod and ranged between 81 ? 63 and 284 ? 52 mg gI wk71 (regression coefficient ? standard error; n = 30). In general, total chlorophyll (a+b) was greater for dioecious than for monoecious plants. The ratios of chlorophyll "a" to chlorophyll "b" and carotenoids to chlorophyll a changed with increasing photoperiod and plant age in a similar manner for both monoecious and dioecious plants. Additional index words. Tuber production, aquatic weed, weed ecology, dry matter allocation, HYLLI.

Abundance of Pithophora oedogonia akinetes displayed seasonal changes, being greatest in winter and lowest in summer. Akinete abundance showed significant (P < 0.001) negative correlations with photoperiod(r = -0.53) and water temperature (r= -0.75) during the period February 1978 through June 1979. Experiments in which akinete germination was studied in response to manipulations of nutrients (NO3-N and PO4-P), photoperiod and temperature indicated that temperature was the primary factor regulating the timing of the vernal flush of akinete germination observed in Surrey Lake.

Photoperiods of 10 or 12 h enhanced vegetative propagule production by Potamogeton nodosus Poir. and P. pectinatus L. The proportion of plants producing vegetative propagules and the number and weight of propagules per plant increased over time, and were greatest after 8 weeks. Maximum vegetative propagule production was 12±6 tubers per plant (mean±S.D.; n =7) for P. pectinatus and 5±3 for P. nodosus . Tuber or winter-bud dry weight accounted for 38 or 27% of total plant dry weight, respectively, for plants grown for 56 days under a 10-h photoperiod. Vegetative propagules appeared to be produced instead of new shoot tissue under short-day conditions. Chlorophyll a and carotenoid content were lower for P. nodosus at short photoperiods, but not for P. pectinatus . The changes in chlorophyll a : b and carotenoid:ratio chlorophyll a suggest that synthesis/degradation of these pigments is tightly coupled and that short photoperiods may initiate senescence in P. nodosus .

Glasshouse competition experiments with Hydrilla verticillata (L.f.) Royle indicate that plants grown from turions are weaker competitors than those grown from tubers, when compared to the widely distributed macrophyte, Potamogeton pectinatus L. These results support an earlier hypothesis about the importance of propagule size for predicting the outcome of plant competition (Grace 1985; Schaffer and Gadgil 1980). Results of outdoor growth experiments indicate that even though Hydrilla plants from turions are relatively weaker competitors, they are able to grow succesfully in an existing macrophyte bed composed of either, P. pectinatus or P. gramineus. During the early stages of Hydrilla invasion into an area of existing macrophytes, native plants may coexist with Hydrilla. However, once the abundance of Hydrilla tubers in the sediment increases, Hydrilla may displace existing plants.

We present here the nucleotide sequence of the small subunit (18S) rRNA gene from wheat mitochondria. Aside from five discrete variable domains, this gene and the analogous (16S) rRNA gene in Escherichia coli show essentially a one-to-one correspondence in their potential secondary structures, with regions accounting for 86% of the bacterial 16S rRNA having a strict secondary structure counterpart in the mitochondrial 18S rRNA. Primary sequence identity between the two rRNAs ranges from 73% to 85% (76% overall) within regions of conserved secondary structure. Within a smaller secondary structure core common to all small subunit rRNAs, the wheat mitochondrial sequence shares substantially more primary sequence identity with the E. coli (eubacterial) sequence (88%) than with the small subunit rRNA sequences of Halobacterium volcanii (an archaebacterium) (71%) or Xenopus laevis cytoplasm (61%). Moreover, the wheat mitochondrial sequence contains a very high proportion of certain lineage-specific residues that distinguish eubacterial/plastid 16S rRNAs from archaebacterial 16S and eukaryotic cytoplasmic 18S rRNAs. These data establish that the ancestry of the wheat mitochondrial 18S rRNA gene can be traced directly and specifically to the eubacterial primary kingdom, and the data provide compelling support for a relatively recent xenogenous (endosymbiotic) origin of plant mitochondria from eubacteria-like organisms.

Mitochondrial DNA (mtDNA) is known to encode the distinctive ribosomal RNA (rRNA) species found in the organelle-specific ribosomes that function in mitochondrial protein synthesis [1]. Qualitatively and quantitatively, rRNA production is a major genetic function of mtDNA, and the factors controlling mitochondrial rRNA (mt-rRNA) synthesis are therefore of great interest. Moreover, since homologous rRNA species are found in all systems (eukaryotic cytosol, prokaryotic, chloroplast, mitochondrial) having a ribosome-based translation system, comparative studies of these rRNAs and their genes are an important means of illuminating the evolutionary history of the eukaryotic cell and its organelles [2]. For these reasons, we have been investigating the organization and evolution of the rRNA genes in plant mitochondria, particularly those of wheat, Triticum aestivum.

Nucleotide sequences of nine eukaryotic and nine eubacterial 5S rRNAs have been selected for their diversity and subjected to analysis of primary and potential secondary structure. This analysis has allowed the quantitative confirmation of several previously made observations concerning 5S rRNA structure: (i) these two 5S rRNAs are derived from a common ancestor and probably perform essentially the same function in protein synthesis; (ii) one domain of 5S rRNA has undergone considerable divergence of structure (and presumably function) since the separation of the eukaryotic and eubacterial lineages; and (iii) single-stranded regions are more highly conserved than double-stranded regions. In addition, this analysis leads us to propose that (i) some of the highly conserved nucleotide residues in single-stranded regions interact in a specific manner with protein components of the translational apparatus, and (ii) repetitive folding and unfolding of helical regions occurs in two regions of eukaryotic 5S rRNA and one region of eubacterial 5S rRNA. In the context of these observations and propositions we also consider the potential secondary structure of plant mitochondrial 5S rRNA.Nucleotide sequences of 5.8S rRNAs have yielded less information about secondary structure and possible functional interactions. However, we have identified highly conserved and variable regions within this molecule and we show (in contrast to the situation with 5S rRNA) that these do not correlate well with proposed single-stranded and helical regions in a current model of 5.8S secondary structure.

In Crithidia fasciculata, a trypanosomatid protozoan, the nuclear-encoded `28S' rRNA is multiply fragmented, comprising two large (c and d) and four small (e, f, g and j) RNA species. We have determined that the coding sequences for these RNAs (and that of the 5.8S rRNA, species i) are separated from one another by spacer sequences ranging in size from 31 to 416 bp. Coding and spacer sequences are presumably co-transcribed, with excision of the latter during post-transcriptional processing generating a highly fragmented large subunit (LSU) rRNA. Secondary structure modelling indicates that the C. fasciculata LSU rRNA complex (seven segments, including 5.8S rRNA) is held together in part by long-range intermolecular base pairing interactions that are characteristic of intramolecular interactions in the covalently continuous LSU (23S) rRNA of Escherichia coli. At least one functionally critical region (encompassing the α-sarcin cleavage site) is contained in a small RNA species (f) rather than in one of the two large RNAs. Within a proposed secondary structure model of C. fasciculata LSU rRNA, discontinuities between the different segments (created by spacer excision) map to regions that are highly variable in structure in covalently continuous LSU rRNAs. We suggest that `rRNA genes in pieces' and discontinuous rRNAs may represent an evolutionarily ancient pattern.