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The actin gene family of Nicotiana tabacum has been partially characterised by Southern hybridisation and by isolating lambda EMBL4 recombinants from a genomic library having homology to the soybean actin gene, Sac3. The number of actin genes with homology to Sac3 is estimated at between 20 to 30, based on Southern hybridisation and library screening, though the total gene family may be larger. Twenty-four recombinant lambda clones were isolated, 18 had unique restriction profiles and from these, 2 clones, Tac9 and Tac25, were selected for further study. The region of Tac25 hybridizing to Sac3 was sequenced and shown to contain an open reading frame (ORF) with homology to actin. Partial sequencing of Tac9 revealed a sequence with homology to the third exon of Tac25 and Sac3. The two tobacco actin sequences were compared to other reported actin gene sequences; Tac25 was closely related to the allelic potato actins, Pac58 and Pac85, while Tac9 was more related to Pac79 than to other plant actins. Northern hybridisation analysis showed that while Tac9 detected actin transcripts in RNA from root, leaf, stigma and pollen, Tac25 transcripts were only detected in pollen RNA.

Previous studies have shown that c-Jun NH(2)-terminal kinase (JNK) belongs to the mitogen-activated protein kinase (MAPK) family of signal transduction components that are rapidly initiated and activated by many extracellular stimuli. However, the potential role of JNK in mediating tumor promotion and carcinogenesis is unclear. We show here that in JNK2-deficient (Jnk2(-/-)) mice, the multiplicity of papillomas induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) was lower than that in wild-type mice. Papillomas on wild-type mice grew rapidly and were well vascularized compared with Jnk2(-/-) mice. After the 12th week of TPA treatment, the mean number of tumors per mouse was 4.13-4.86 in wild-type mice but only 1.13-2.5 in Jnk2(-/-) mice. TPA induced phosphorylation of extracellular signal-regulated kinases and activator protein-1 DNA binding activity in wild-type mice, but the phosphorylation of extracellular signal-regulated kinases and activator protein-1 DNA binding were inhibited in Jnk2(-/-) mice. These data suggest that JNK2 is critical in the tumor promotion process.

The wheat insertion sequence Wis 2-1A possesses all the structural features characteristic of retrotransposons. Its long-terminal repeats (LTRs) are unusually long (1,755 bp) compared with those of other retrotransposons. Sequence analysis revealed that they differ from each other by only six point mutations. They contain a few tandem direct repeats, which could be explained by slippage mechanisms during replication. Almost half (44%) of the length of the LTRs is occupied by hairpin structures, which may relate to their large size. Possible origins of these inverted repeats are proposed, including the insertion and imprecise excision of transposable elements and errors when the DNA replication intermediate switches RNA template during retrotransposon replication.

The epidermal-growth-factor (EGF) receptor was expressed in the human erythroleukaemic cell line K562 by transfection of the receptor cDNA. EGF-receptor biochemistry appears altered in the K562 transfectants. Autophosphorylation of the K562 receptor is not stimulated substantially by EGF. Tyrosine kinase activity of the receptor is high in the absence of EGF, whereas receptor affinity for EGF is low. K562 cells are shown to lack mRNA for transforming growth factor alpha (TGF alpha). Therefore autocrine stimulation of the K562 receptor, at least by TGF alpha, does not explain the observed receptor biochemistry. The K562 receptor is phosphorylated at a single major site in intact cells, a threonine residue that may be Thr-669. Possible mechanisms of regulation of the EGF receptor in the K562 transfectants are discussed.

Mitochondrial DNAs have been examined in accessions of 25 Mexican races of maize and compared with the mitochondrial DNAs previously found in inbred lines from the USA. Many variants were found. Low molecular weight DNA components, not previously found in US lines, were found in many of the accessions. Accessions classified as belonging to the same race, and plants from a single accession, sometimes had different mitochondrial genomes. Mitochondrial genomes similar to those in T and S cytoplasms were found in Mexican accessions.A low molecular weight linear DNA species has partial homology with a sequence in the high molecular weight mitochondrial genome. All plants with a shorter version of the linear molecule had a correspondingly altered region of homology in the high molecular weight genome.There is evidence that the geographical distribution of mitochondrial DNA types within Mexico is not random. One type, found in the oldest races, appears to be widely dispersed but another less common type appears to be confined largely to coastal regions. The potential value of these findings in maize breeding and for evolutionary studies is discussed.

Mitochondrial DNA from male-sterile lines of maize carrying S cytoplasm contains two small DNA species which are absent from N (fertile) and other male-sterile cytoplasms. Portions of these species have been purified and amplified by constructing recombinant plasmids in vitro. Probes made with these plasmids have been used to demonstrate; i) a homologous region in the N mitochondrial genome, which may indicate the origin of the S specific DNA species. ii) two other DNA species present in low amounts in S cytoplasm only. iii) the absence of strong homology to the S specific DNA species in mitochondria from C and T male-sterile cytoplasms.

A simple repeating unit of the rye genome, previously known in tandem arrays, has now been shown to occur also in more complex repeating units. One such unit has been cloned and found to contain some sequences unrelated to the simple repeat. It is proposed that tandem arrays of complex repeats are often produced by amplification of DNA segments formed by recombination of different repeats and possibly unique sequences. Some complex variants of repeats are not in high copy number on all rye chromosomes.

The relationship between the chromosomal location of heterochromatin C-bands and of four non-homologous repeated sequence families constituting 8 to 12% of total rye DNA has been investigated in chromosomes of rye (Secale cereale) by in situ hybridisation. Three rye varieties, a set of rye disomic additions to wheat and a triticale were studied. Only centromeric and nucleolar organizer region (NOR) associated C-bands failed to display hybridisation to at least one of the sequences and many telomeric blocks of heterochromatin contained all four repeated sequence families. Both between-variety differences in the chromosomal distribution of repeated sequences, and intravarietal heterozygosities were frequently noted and are probably widespread. — Previously reported deletions of heterochromatin from King II rye chromosomes added to the Holdfast wheat complement were correlated with deletions of some, but not all, of the highly repeated sequence families. A previously unreported loss of some families from King II rye chromosome 4R/7R in a Holdfast wheat genetic background was detected. This loss was not associated with complete deletion of a C-band. A deletion has also probably occurred from the short arm telomere of 4R/7R in the triticale variety Rosner. It is suggested that the families of repeats in rye telomeric heterochromatin which are absent from wheat are selected against in the wheat genetic background.

Two pairs of chromosomes (1U and 5U) in Aegilops umbellulata possess ribosomal RNA genes. This has been proven by studying wheat plants into which 1U and 5U chromosomes have been introduced separately. These plants have more ribosomal RNA genes than the recipient wheat plants and additional clusters of rDNA when examined by in situ hybridisation. The repeating rDNA unit in Aegilops umbellulata is longer than most of the units in the wheat variety Chinese Spring, the additional DNA probably being in the non-transcribed spacer. This was determined from restriction endonuclease maps of rDNA. In Chinese Spring plants possessing 1U or 5U chromosomes, the largest nucleoli are formed on 1U or 5U chromosomes and the wheat nucleolus organisers form micronucleoli. This is not because the nucleolus organisers on chromosomes 1U and 5U have many more rRNA genes than wheat nucleolus organisers. It is suggested that the Aegilops umbellulata nucleolus organisers are dominant over those of wheat because they compete more effectively for some limiting factor. — The partial inactivation of the wheat nucleolus organisers by chromosomes 1U or 5U does not result in a reduced total nucleolus volume in root tip or pollen mother cells, because of the compensation by the nucleolus organisers of chromosomes 1U or 5U. The amount of RNA in seedlings is not markedly affected by the partial inactivation of the wheat nucleolus organisers.

cDNA clones encoding wheat HMW glutenin subunits have been isolated from a cDNA bank made to poly A(+) RNA from developing wheat endosperm var. Chinese Spring. One such clone, pTag 1290, has enabled us to identify the HMW glutenin mRNA species. The DNA sequence of this clone has been partially determined and it contains several tandem DNA repeats. The sequence is discussed in relation to the generation of the HMW glutenin subunit gene family. Analysis of the organization of the HMW glutenin sequences in the wheat genome revealed that the genes encoding HMW glutenin subunits exist in low copy number and are located on the long arm of each of the homoeologous group 1 chromosomes.

The expression of a wheat genomic clone containing the entire coding sequence of the high molecular weight glutenin subunit 12 gene flanked by 2.6 kilobases of 5' and 1.5 kilobases of 3' sequences has been studied after introduction into tobacco. Seeds of different tobacco plants containing the full-length wheat genomic clone accumulated different amounts of intact high molecular weight glutenin subunit mRNA and of a polypeptide displaying the solubility, molecular weight, and antigenic properties of the high molecular weight glutenin subunit 12. The wheat protein accumulated without obvious degradation products and constituted up to approximately 0.1% of the total tobacco endosperm protein. Restriction fragments corresponding to 2.6 kilobases, 1.4 kilobases, and 433 base pairs of high molecular weight glutenin 5' upstream sequence were fused to the coding sequence of the chloramphenicol acetyltransferase (CAT) gene in the vector polyCATter and transferred into tobacco. Chloramphen- icol acetyltransferase enzyme activity was detected only in the seed endosperm tissue of the transformed plants. It was detected in tobacco seeds 8 days after anthesis and persisted until seed maturity. It is concluded that 433 base pairs of high molecular weight glutenin upstream sequence are sufficient to confer endosperm-specific expression of this monocot gene in the dicot tobacco.

The environment has been shown to induce heritable changes in three plant systems, namely flax, Nicotiana rustica, and soybean cell suspension cultures. Changes in nuclear DNA sequences associated with the induction phenomenon have been demonstrated in both flax and soybean. In flax, which has been most extensively studied, the overall variation in the nuclear DNA is described together with a number of transcribed and nontranscribed sequences which have been shown to vary. Possible mechanisms by which the observed DNA changes could arise are discussed and the phenomenon of environmentally induced DNA variation compared with examples of gene amplification or deletion in other systems.

The DNAs of wheat and rye plants with rye B chromosomes have been compared with wheat, rye and oats DNAs by DNA/DNA hybridisation. The presence of DNA from B chromosomes made no significant difference to the proportion of repeated sequence DNA. The repeated sequence fractions of these cereal DNAs were quantitatively divided into eight different groups on the basis of the amount of DNA/DNA hybridisation occurring between the different DNAs. Rye A and B chromosomes contained similar proportions of three of the groups. These results, together with estimates of the thermal stabilities of all the renatured DNA duplexes suggest that rye B chromosome DNA is very similar to rye A chromosome DNA in the proportion and heterogeneity of its repeated sequences.

Molecular analysis of the heterodimeric T-cell antigen receptor of insulin-specific class II-restricted T-cell hybridomas (THys) derived from C57BL/6 (B6) wild-type and B6.C-H-2bm12 (bm12) mutant mice revealed that such T cells use a diverse V gene repertoire. Analysis of three THys that use related V genes, however, showed a number of novel features. Two THys that share major histocompatibility complex restriction use V alpha genes that are 98.6% homologous. Two THys sharing the same antigen fine specificity use a particular germ line V beta D beta J beta combination. A 21-base-pair deletion in the 5' segment of the J beta gene occurs in one THy, suggesting a novel mechanism for generating diversity in T-cell antigen receptor beta genes. The first amino acid encoded by N sequences at the V-D junction is conserved in a pair of T cells which recognize identical antigenic epitopes. The implications of these findings for the structural mechanisms underlying major histocompatibility complex-restricted antigen-specific T-cell recognition are discussed.

In this paper we describe the physical properties, sequence divergence and chromosomal distribution of six different repeated sequences in Secale cereale (cultivated rye). All these sequences are located predominantly within blocks of constitutive telomeric heterochromatin which can be observed on all seven pairs of chromosomes of this species by Giemsa staining. The telomeric heterochromatin accounts for 12–18% of the DNA of S. cereale but less than 9% in S. silvestre. Four of the characterized repeats account for most, if not all, of this difference in telomeric heterochromatin DNA amount between the closely related species. These four repeats are present in high copy number in S. cereale DNA but are not detected by various techniques in S. silvestre DNA. The remaining two characterized repeats are common to the two Secale species. Both are arranged in simple tandem arrays with a repeating unit of 120 bp. The major common repeat represents approximately 2.4% of S. cereale DNA and the minor one represents less than 0.2%. The major common repeat is localized in interstitial sites in Secale chromosomes as well as in the telomeric sites. The four S. cereale-specific repeats have several distinguishing physical properties beside their absence in S. silvestre. They are complex. Each contains simple subrepeats interspersed with an unrelated sequence without subrepeats. The major repeat, constituting 5–6% of S. cereale DNA, has a repeat length of 480 bp consisting of a doublet of ∼140 bp interspersed with an unrelated sequence of approximately 230 bp. Another S. cereale repeat representing 2–3% of the total DNA has a repeat unit of 610 bp consisting of a triplet of 110 bp units interspersed with a 280 bp unrelated sequence. Both the 480 and 610 bp repeat families are more homogeneous in sequence, as judged by the thermal stability of duplexes formed in vitro, than the repeat families found in both Secale species. The two remaining S. cereale-specific repeats are related to the minor 120 bp common repeat. One had a repeat unit of 356 bp and consists of two of the 120 bp subrepeats interspersed with an unrelated sequence of 116 bp. The other, a 630 bp repeat, has a 270 bp sequence interspersed with three of the 120 bp subrepeats. We suggest that each of the S. cereale-specific repeats may have evolved by the insertion of DNA elements into an array of simple repeats followed by amplification of the portion of the array containing the inserted sequence.

A DNA copy (cDNA) of the common European strain (strain-group 3) potato virus X RNA (PVX3-RNA) was prepared by reverse transcription of PVX-RNA and amplified by cloning into E. coli plasmid pBR322. This DNA hybridizes with PVX-RNA and not at all to RNA of potato virus Y. A low amount of cross-hybridization was observed with strain-group 2 of PVX (PVX2). However, by hybridization of PVX2-RNA to restriction endonuclease fragments of PVX3-cDNA, it was demonstrated that cross-homology was not present in all regions of the PVX genome. The cloned PVX3-cDNA was hybridized to viral RNA in crude sap samples which had been spotted on to nitrocellulose membrane. The signal, which was assayed by autoradiography, allowed the detection of 50 pg of viral RNA. This value was not adversely affected by encapsidation of the RNA, nor by components of crude sap. The sap spot hybridization was as effective as enzyme-linked immunosorbent assay both in detection of small quantities of purified PVX3, and in screening of plants from a breeding programme for immunity to PVX3 infection.

The amounts of a 1.9 kb mitochondrial plasmid relative to sequences in another mitochondrial DNA replicon and also to nuclear ribosomal DNA sequences have been compared in maize leaves and anthers. Similar comparisons have been made between plants with the same nuclear genotype but containing normal, S, or T cytoplasms. The ratio of 1.9 kb plasmid to nuclear rDNA is lower in plants with normal cytoplasm than in plants with S or T cytoplasm. It also differs between leaves and anthers. Furthermore, the relative concentration of the mitochondrial DNA sequences belonging to different replicons differs between leaves and anthers. It is concluded that components of different mitochondrial replicons are not maintained in fixed ratios during development and that the concentration of the 1.9 kb plasmid is regulated, in part, by cytoplasmically-inherited determinants. The 1.9 kb plasmid is absent from lines with the Vg cytoplasm, but related sequences are found in the maize nuclear genome.

In hexaploid wheat and diploid rye, different varieties have different numbers of ribosomal RNA genes as indicated by rRNA/DNA hybridisation. Wheat has four different chromosomes which carry nucleolar organisers. Analyses of DNA isolated from substitution lines in which each of these nucleolar organiser chromosomes of several varieties has been substituted one at a time into a common genetic background, have indicated that none of the four organiser chromosomes possess an invariant number of ribosomal RNA genes. The ribosomal RNA gene complement of the varieties investigated can be approximately accounted for by the sum of the ribosomal RNA genes on each of the four nucleolar organiser chromosomes.

1. Mitochondrial DNA from Tetrahymena pyriformis strain T has a buoyant density (rho) of 1.685 compared with rho1.688 for whole cell DNA. Mitochondrial preparations from T. pyriformis strain W show an enrichment of a light satellite (rho1.686), although this is not obtained free from nuclear DNA (rho1.692). 2. T. pyriformis mitochondrial DNA renatures rapidly and the kinetics of this process indicate a complexity of approx. 3x10(7) daltons. 3. The base-pairing in the renaturation product is of a precise nature, since the ;melting' temperature (80.5 degrees C) is indistinguishable from that of the native DNA (80.5 degrees C). 4. Centrifugation of mitochondrial DNA in an alkaline caesium chloride density gradient gives two bands, implying the separation of the complementary strands.

The use of in situ hybridization to identify chromosomal regions complementary to RNA or DNA sequences is now over 10 years old (1). In plants, the technique has been used to identify the chromosomal location of a limited number of highly repeated sequences but with few exceptions the nucleic acid probe molecules were impure, therefore limiting the resolution and the interpretations. The advent of the purification of DNA sequences by the molecular cloning of DNA fragments in bacterial plasmids has, however, opened up the possibility of obtaining large numbers of pure probes, thereby making in situ hybridization potentially much more useful to cytogeneticists and others interested in chromosome structure and chromosome identification.

We have analyzed DNA modification in the human gamma delta beta-globin gene region at 17 cleavage sites of restriction endonucleases which are unable to cleave DNA if 15-methylcytosine is present at certain positions in their respective cleavage sites. Using this criterion, all sites tested in the globin gene region are fully modified in the germ line (sperm) DNA. In somatic tissues, however, methyl groups are absent at specific sites in the globin gene region. In tissues not expressing the genes, these losses range from one of these cleavage sites in lymphocyte DNA to essentially all of these sites in the entire region in placental DNA. In the DNA of tissues expressing the globin genes, the region surrounding and including the genes expressed shows a low level of modification, whereas the neighboring DNA regions have a high level of modification. The data suggest that a low level of DNA methylation may be a necessary, but not a sufficient, condition for gene expression in higher eucaryotes.

Molecular cloning technology has introduced exciting ways of studying eukaryotic chromosomes. The purpose of this paper is to illustrate ways in which molecular cloning has enabled us to study components of plant chromosomes.

Reassociation kinetics of sheared hexaploid wheat DNA in 0.18 M Na+ at 60 °C show the presence of three major classes of nucleotide sequences; (1) a very rapidly reannealing fracton that comprises about 4 to 10% of the genome and reanneals virtually instantly. This fraction contains sequences which appear to be randomly distributed through at least 40% of the genome; they may be palindromic sequences; (2) a heterogeneous intermediate reannealing fraction comprising about 70–80% of the genome which consists of families with apparent repetition frequencies ranging from about 100–100000; and (3) a slow reannealing fraction. These fractions have been isolated and studied separately. Seventy percent of the slow reannealing fraction (12 to 20% of the genome) was found to contain sequences present in approximately six copies per hexaploid genome. The single copy sequences in the three constituent diploid genomes of hexaploid wheat appear to show near complete homology to one another.

Although protoplasts of cereals and grasses have proved difficult to manipulate in culture, mature plants have recently been regenerated from protoplasts of rice (Abdullah et al., 1986), maize (Chai et al., pers. comm.) and Polypogon (Chan and Xia, 1987). These results are encouraging, not only for protoplast morphogenesis but also for the transformation of cereals and grasses. We report protoplast culture and transient expression of foreign genes in the grass Echinocloa colonum.

Mitochondrial DNA preparations were made from 31 maize lines carrying different sources of cytoplasm in the same nuclear genetic background. The DNAs were analyzed by agarose gel electrophoresis. A number of discrete low molecular weight bands were present in all lines. However, only four different DNA banding patterns were observed. These were correlated with the N, T, S and C cytoplasms defined by nuclear fertility restorer genes. Of the 31 cytoplasmic sources examined, six possessed DNA species characteristic of N cytoplasms, four possessed DNA species characteristic of T cytoplasm, 19 possessed DNA species characteristic of S cytoplasm and two possessed DNA species characteristic of C cytoplasm. This classification is in complete agreement with that based on mitochondrial translation products reported in the accompanying paper. No within-group heterogeneity was observed in the DNA banding patterns, indicating a lack of cytoplasmic variation within the four cytoplasmic groups. Attributes of the various methods available for classifying maize cytoplasms are compared and discussed.

We have constructed a physical map of restriction endonuclease cleavage sites in the (delta (+) beta)-globin gene region in the DNA of patients with (delta beta(0))-thalassaemia. This map shows that a 10 kb deletion has occured in (delta beta (0))-thalassaemia to remove the entire beta-globin gene and the 3' portion of the delta-globin gene. The 5' terminus of the deletion is in the large intron of the delta-globin gene and the 3' terminus 1.8 kb to the 3'-side of the beta-globin gene. A similar deletion of about 7 kb has been described previously in the DNA of patients with Hb Lepore; the 5' terminus of the deletion is also in the delta-globin gene but the 3' terminus is in the beta-globin gene. Comparison of the foetal (gamma) globin gene expression in adults with (delta beta(0))-thalassaemia and Hba Lepore suggests that the 3' extragenic regions of the beta-globin gene contain DNA sequences involved in the regulation of gamma-globulin gene expression.

The isolation and characterisation by DNA sequencing of a low molecular weight (LMW) glutenin gene from wheat is described. The deduced protein contains a signal peptide, a central repetitive region rich in proline and glutamine and N and C terminal non-repetitive domains, similar to other prolamins. A detailed comparison of the C terminal domain of 20 prolamin genes enabled us to divide them into 4 families. The LMW glutenin family is distinct from the alpha, beta- and gamma-gliadin families of wheat and is closest to the B hordein genes of barley. This and other comparisons were also used to assess the pattern of genetic variation among prolamin sequences and to provide a molecular basis for the interpretation of prolamin size polymorphism. The 5' flanking fragment of the isolated gene was previously shown to direct endosperm-specific expression of a reporter gene in transgenic tobacco. Evidence is provided that the isolated gene is also active in wheat and its transcription initiation site was determined. Features of the gene which may be relevant to its activity are discussed.

Previous work by Jacobson et al. (1) has shown that the number and distribution of (dA.dT)25 tracts in the nuclear DNA of the slime mold Dictyostelium discoideum reflects the number and the distribution of transcriptional units. To investigate whether this is a general phenomenon we compared the nuclear DNAs of other primitive eukaryotes with respect to their content of large (dA.dT)n tracts via the thermal stability of their hybrids with poly(rA). The results of our analysis indicate that all nuclear DNAs tested have (dA.dT)20-25 tracts, but the frequency of such tracts varies from one per 5.4 X 10(6) daltons in Dictyostelium nuclear DNA to one per 2.8 X 10(8) daltons in Crithidia luciliae nuclear DNA. We conclude that the presence of (dA.dT)20-25 tracts is not an obligatory characteristic of the transcriptional unit in primitive eukaryotes. Chromatography of native DNAs on poly(rU) Sephadex columns shows that the large (dA.dT)n tracts occurring in the genomes of both primitive and higher eukaryotes are widely distributed throught these genomes.

One of the most bewildering features about the nuclear DNA of higher plants is the variability in amount and type between different species. The haploid DNA contents of angiosperms range from about 0.5 to over 200 pg (Bennett and Smith 1976). Sometimes the Variation is extensive within a genus. For example, there is a threefold Variation in haploid DNA content within the genus Lathyrus without any Variation in chromosome number (Rees and Hazarike 1969) and a tenfold Variation in the genus Crepis (Jones and Brown 1976). The amount of DNA in Angiosperms at the lowest end of the range, i. e., about 0.5 pg is approximately five times the amount in Drosophila melanogaster, a fact which immediately provokes the conclusion that higher plants contain a considerable excess of DNA over what is needed to specify their biological complexity (Hinegardner 1976). The large amounts of DNA in plants and the extensive Variation in DNA content between species pose many problems of a technical as well as an intellectual nature for research on chromosomal DNA sequences.

The sporophytic self-incompatibility system of Brassica species is controlled by a single locus, S. Recognition of self between pollen and stigma is probably mediated by S locus-specific glycoproteins (SLSGs). We describe the isolation, from an S29 homozygote of Brassica oleracea, of two different cDNA clones for transcripts which are equally abundant in stigmas competent for self-incompatibility and each of which is homologous to previously reported SLSG sequences. Extensive DNA sequence divergence between the two clones precludes their cross-hybridisation and each acts as a gene-specific probe. All S genotypes appear to have a single copy of each gene but there are significantly different levels of polymorphism associated with each. The clear structural homology between the two indicates a gene duplication involving the S locus and, perhaps, related to the evolution of self-incompatibility.

Repeated sequences, by their very nature of being present in many copies in the genome, appear to be prone to rapid change. Enzymes, especially those involved in recombination and replication, appear to mistake one copy for another and produce various new DNA structures. Other sorts of repeats, notably those in transposable elements, are recognised by enzymes, excised from the chromosomes and integrated elsewhere.

The presence of rRNA genes on homoeologous chromosomes 1A, 1B, and 1D of hexaploid wheat was investigated by rRNA/DNA hybridization, using DNA purified from aneuploid and substitution line derivatives of the variety Chinese Spring. Doubling the number of 1B chromosomes increased the number of rRNA genes by 31–49% but deleting the 1B chromosomes decreased the number by only 15–23%. This suggests that changes may occur in rRNA gene multiplicity at other nucleolar organizer sites to partially compensate for a deficiency of rRNA genes. There was no unequivocal evidence of rRNA genes on Chinese Spring chromosome 1A or 1D, but other varieties were shown to have rRNA genes on chromosome 1A. These results are consistent with the cytological observations that chromosomes 1A and 1B but not 1D possess nucleolar organizers.

A region upstream of the murine major histocompatibility complex gene, E alpha d, has been shown previously to be required for B-cell expression. Binding of the B-cell-specific factor, NF-kappa B, to a site within this region is indistinguishable from that observed with the kappa enhancer binding site. NF-kappa B may be responsible for E alpha d B-cell expression.