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Rickettsia prowazekii, the causative agent of epidemic typhus, is an obligate, intracellular, parasitic bacterium that grows within the cytoplasm of eucaryotic host cells. Rickettsiae exploit this intracellular environment by using transport systems for the compounds available in the host cell's cytoplasm. Analysis of the R. prowazekii Madrid E genome sequence revealed the presence of a mutation in the rickettsial metK gene, the gene encoding the enzyme responsible for the synthesis of S-adenosylmethionine (AdoMet). Since AdoMet is required for rickettsial processes, the apparent inability of this strain to synthesize AdoMet suggested the presence of a rickettsial AdoMet transporter. We have confirmed the presence of an AdoMet transporter in the rickettsiae which, to our knowledge, is the first bacterial AdoMet transporter identified. The influx of AdoMet into rickettsiae was a saturable process with a K(T) of 2.3 micro M. Transport was inhibited by S-adenosylethionine and S-adenosylhomocysteine but not by sinfungin or methionine. Transport was also inhibited by 2,4-dinitrophenol, suggesting an energy-linked transport mechanism, and by N-ethylmaleimide. AdoMet transporters with similar properties were also identified in the Breinl strain of R. prowazekii and in Rickettsia typhi. By screening Escherichia coli clone banks for AdoMet transport, the R. prowazekii gene coding for a transporter, RP076 (sam), was identified. AdoMet transport in E. coli containing the R. prowazekii sam gene exhibited kinetics similar to that seen in rickettsiae. The existence of a rickettsial transporter for AdoMet raises intriguing questions concerning the evolutionary relationship between the synthesis and transport of this essential metabolite.

Previous studies showed that high concentrations of methionine (> 1 mM) inhibited aggregation and fruiting body formation in Myxococcus xanthus (E. Rosenberg, D. Filer, D. Zafriti, and S. H. Kindler, J. Bacteriol. 115: 29-34, 1973, and J. M. Campos and D. R. Zusman, Proc. Natl. Acad. Sci. USA 72:518-522, 1975). However, the mechanism for the inhibition was unclear. In this study, we found that high levels of methionine inhibited the biosynthesis of S-adenosylmethionine (SAM) and that reduced intracellular levels of SAM are correlated with defective chemotactic movements and reduced developmental gene expression. In addition, we found that methionine analogs and high concentrations of amino acids which are known to affect SAM synthesis in other bacteria, such as threonine, lysine, and isoleucine, also caused reduced cellular levels of SAM and blocked fruiting body formation in M. xanthus. These results indicate that SAM is required for development of M. xanthus and the inhibitory effect of methionine on development results, at least in part, from its blocking of the biosynthesis of SAM.

As a first step toward developing the genetic potential of extremely thermophilic archaebacteria, mutant strains of Sulfolobus acidocaldarius were selected by plating cells directly on solid medium containing one of several growth inhibitors. Three spontaneous resistance phenotypes were observed (5-fluorouracil resistance, novobiocin resistance, and L-ethionine resistance), each at a different average frequency. Characterization of representative strains showed each of the three mutant phenotypes to provide a potentially useful genetic marker.

Mutants of Spirulina platensis resistant to 5-fluorotryptophan, beta-3-thienyl-alanine, ethionine, p-fluorophenylalanine, or azetidine-2-carboxylic acid were isolated. Some of these mutants appeared to be resistant to more than one analog and to overproduce the corresponding amino acids. A second group was composed of mutants that were resistant to one analog only. Of the latter mutants, one resistant to azetidine-2-carboxylic acid was found to overproduce proline only, whereas one resistant to fluorotryptophan and one resistant to ethionine did not overproduce any of the tested amino acids.

Plectonema boryanum mutants that are resistant to ethionine are unable to incorporate ethionine into acid-precipitable material. Ethionine causes bleaching of chlorophyll in sensitive cells.

A pleitropic ethionine-resistant mutant of Neurospora contains an S-adenosylmethionine synthetase that is labile to heat and dialysis but exhibits normal kinetics for methionine and ethionine.

S-Adenoxyl-L-methionine (SAM) inhibited leucine transport in Saccharomyces cerevisiae. By using a mutant defective in the active transport of SAM, we demonstrated that the inhibitory effect was exerted at an extracellular site. Cells preincubated wtih SAM for 120 min became refractory to its inhibitory effect, which was not a result of either the active transport or the metabolism of SAM. The quantitative recovery of labeled SAM from the incubation medium indicated that SAM, and not a metabolite, was the true inhibitory molecule. S-Adenosyl-L-homocysteine and S-adenosyl-L-ethionine also functioned as inhibitors of leucine transport, whereas S-adenosyl-D-methionine, S-adenosyl-D-homocystein, 5'-methylthioadenosine, 5'-dimethylthioadenosine, and adenosine lacked this property. Kinetic studies demonstrated that SAM was a competitive inhibitor of leucine transport.

Starting with a relaxed (relA) strain, mutants with reduced activity of adenosine triphosphate:L-methionine S-adenosyl transferase (EC 2.5.1.6; SAM synthetase) were isolated in Bacillus subtilis. One such mutant (gene symbol metE1) had only 3% of the normal SAM synthetase activity but grew almost as well as the parent strain. Another mutant was isolated (gene symbol spdC1) as being able to sporulate continually at a high frequency; it had one-half the normal SAM synthetase activity at 33 degrees C. Both mutants continually and spontaneously entered spore development at a higher frequency than the parent strain in a medium containing excess glucose, ammonium ions, and phosphate. Sporulation was prevented by a high concentration of SAM (1 mM or more) or by the combination of adenosine and methionine (0.5 mM or more each), both of which are precursors of SAM. In contrast to this continual increase in the spore titer, addition of decoyinine, an inhibitor of GMP synthetase, rapidly initiated massive sporulation. Various amino acid analogs also induced sporulation in the relA strain, the methionine analogs ethionine and selenomethionine being most effective.

Compared with the parental strain, a Spirulina platensis mutant that is resistant to ethionine incorporated methionine into protein at a reduced rate, whereas ethionine incorporation was practically nil. The methionyl-tRNA synthetase present in crude extracts from the resistant strain showed a reduced affinity for methionine and ethionine.

In contrast to Escherichia coli and Salmonella typhimurium, Bacillus subtilis could convert ethionine to S-adenosylethionine (SAE), as can Saccharomyces cerevisiae. This conversion was essential for growth inhibition by ethionine because metE mutants which were deficient in S-adenosylmethionine synthetase activity, were resistant to 10 mM ethionine and converted only a small amount of ethionine to SAE. Another mutation (ethA1) produced partial resistance to ethionine (2 mM) and enabled continual sporulation in glucose medium containing 4 mM DL-ethionine. This sporulation induction probably resulted from the effect of SAE, since it was abolished by the addition of a metE1 mutation. The induction of sporulation was not simply controlled by the ratio of SAE to S-adenosylmethionine, but apparently depended on another effect of the ethA1 mutation, which could be demonstrated by comparing the restriction of clear plaque mutants of bacteriophage phi 105 grown in an ethA1 strain with the restriction of those grown in the standard strain. The phages grown in the ethA1 strain showed increased protection against BsuR restriction. We propose that SAE induces sporulation through the inhibition of a key methylation reaction.

The effect of three methionine analogues, ethionine, selenomethionine, and trifluoromethionine, on the biosynthesis of methionine in Saccharomyces cerevisiae has been investigated. We have found the following to be true. (i) A sharp decrease in the endogenous methionine concentration occurs after the addition of any one of these analogues to growing cells. (ii) All of them can be transferred to methionine transfer ribonucleic acid in vitro as well as in vivo with, as a consequence, their incorporation into proteins. In the absence of radioactive trifluoromethionine, this conclusion results from experiments of an indirect nature and must be taken as an indication rather than a direct demonstration. (iii) Ethionine and selenomethionine can be activated as homologues of S-adenosylmethionine, whereas trifluoromethionine cannot. (iv) All of them can act as repressors of the methionine biosynthetic pathway. This has been shown by measuring the de novo rate of synthesis of methionine in a culture grown in the presence of any one of the three analogues.

Responses of Halobacterium halobium cells to chemical stimuli have been shown by a capillary technique. Cells were attacted by D-glucose and several amino acids and repelled by phenol. Certain chemicals, such as acetate, benzoate, indole, and NiSO4, that are known to act as repellents of Escherichia coli cells served as attractants for Halobacterium. In the presence of ethionine, sensitivity to attractants was reduced. Arsenate prevented the attraction by glucose without lowering the cellular adenosine 5'-triphosphate level. The ability for chemo-accumulation toward glucose and histidine was interfered with by the formation of photosensory systems. Light-induced motor responses and chemosensory behavior toward glucose and histidine became detectable in the late stationary growth phase only. The behavior toward acetate and indole was not connected to photobehavior in that way: both substances acted as attractants already in the late log phase. Inhibition of bacteriorhodopsin synthesis by L-nicotine allowed chemo-accumulation toward glucose and histidine already in the late logarithmic phase.

An in vitro system was used to study DNA synthesis in lysates of Escherichia coli cells which had been grown in the presence of ethionine. Such lysates showed a reduced capacity to incorporate [3H]TTP into high-molecular-weight material. Activity could be restored by incubation with S-adenosyl methionine and ATP. S-adenosyl methionine-reactivated TTP incorporation required the presence of DNA polymerase I, ATP, and all four deoxyribonucleotide triphosphates. DNA polymerase III was not required.

Mutants of Mycobacterium smegmatis were selected for resistance to ethionine in an effort to obtain methylation-defective strains that were altered in their ability to make methylmannose polysaccharides (MMP) or methylglucose lipopolysaccharides. Two methods were developed for the detection of MMP in cell extracts to aid in the screening for potential mutants, one a qualitative procedure based on iodine binding by the sample after paper chromatography and the other a quantitative procedure based on fluorimetric titration of the MMP with parinaric acid. An ethionine-resistant mutant was obtained that contained only about 25% of the normal level of S-adenosylmethionine and 10% of the normal level of methionine adenosyltransferase (adenosine 5'-triphosphate:L-methionine S-adenosyltransferase, EC 2.5.1.6) activity. When grown in the presence of 0.1% ethionine, the mutant cells contained about 50% of the wild-type levels of methylglucose lipopolysaccharides but only about 7% of the normal level of MMP (wild-type cells contain about 0.14 mM MMP and the mutant contains about 0.01 mM MMP). The amount of fatty acid synthesis in the ethionine-resistant mutant grown in the presence of ethionine was not dramatically altered although the mutant accumulated more short-chain and less long-chain unsaturated fatty acids than the wild-type cells.

The methionine adenosyltransferase is repressed in Saccharomyces cerevisiae during growth in the presence of excess methionine. The relationship of this repression to the level of intracellular S-adenosylmethionine is discussed. In conjunction with these studies, an ethionine-resistant mutant has been investigated which has a low level of methionine adenosyltransferase under all conditions tested. The mechanism of ethionine resistance in the latter strain apparently depends on its inability to form large quantities of intracellular S-adenosylethionine. With respect to the methionine adenosyltransferase, there is no apparent interaction between ethionine-resistant and ethionine-sensitive alleles when both are present in the heterozygous diploid.

Addition of methionine to the growth medium of Escherichia coli K-12 leads to a reduction in the specific activity of S-adenosylmethionine (SAM) synthetase. Thus the enzyme appears to be repressible rather than inducible. Mutant strains (probably metJ(-)) are constitutive for SAM synthetase as well as for the methionine biosynthetic enzymes, suggesting that the regulatory systems for these enzymes have at least some elements in common. Cells grown to stationary phase in complete medium, which have low specific activities of the enzymes, were routinely used for derepression experiments. The lag in growth and derepression when these cells are incubated in minimal medium is shortened by threonine. Ethionine, norleucine, and alpha-methylmethionine are poor substrates or nonsubstrates for SAM synthetase and are ineffective repressors. Selenomethionine, a better substrate for SAM synthetase than methionine, is also slightly more effective at repression than methionine. Although SAM is considered to be a likely candidate for the corepressor in the control of the methionine biosynthetic enzymes, addition of SAM to the growth medium does not cause repression. Measurement of SAM uptake shows that too little is taken into the cells to have a significant effect, even if it were active in the control system.

In a defined medium with added ethionine plus low methionine, phenylalanine, tryptophan, tyrosine, adenine, and additional methionine reversed inhibition of the enteric yeast Candida slooffii by ethionine. Isoleucine and 7-methylguanine restored half-maximal growth. Choline but not triethylcholine inhibited C. slooffii. 6-Mercaptopurine reversed ethionine inhibition and also synergistic inhibition by ethionine plus choline. Protection against ethionine by adenine plus aromatics was also evident with log-phase cells in the absence of methionine. Incorporation of ethionine-ethyl-1-(14)C by resting cells was partially inhibited by aromatic amino acids and methionine. Ethionine depressed incorporation of (3)H-phenylalanine but not of (3)H-adenine. Ethionine-resistant mutants were isolated which incorporated ethionine efficiently and degraded it to yet unidentified substances not including 5'-ethylthioadenosine. Ethionine-sensitive cells accumulated more S-adenosylethionine (SAE) than resistant mutants. Adenine was a good precursor of SAE. Radioactivity from ethionine-ethyl-1-(14)C was recovered from cell fractions of ethionine-sensitive cells with the following distribution: cold trichloroacetic acid-soluble > hot trichloroacetic acid-insoluble > lipids > deoxyribonucleic acid > ribonucleic acid. Total radioactivity recovered from ethionine-sensitive cells was twice as much as that from ethionine-resistant mutants.

An ethionine-resistant mutant of Saccharomyces cerevisiae has been investigated whose mutation (et(r2)) confers resistance to the heterozygous diploid also containing the sensitive allele, et(s). The mutation is apparently specific for reversal of ethionine inhibition. The principal difference between the sensitive et(s) strain and the mutant was the latter's inability to concentrate large intracellular quantities of adenosylethionine. Reduced incorporation of ethyl groups or ethionine in other cellular fractions of the mutant was also detected. The data show that the mutant has not lost the ability to form adenosylethionine. It is suggested that the mutant has an increased ability to hydrolyze this sulfonium compound after it has been synthesized. It is possible that some of the ethionine is detoxified before it can participate in protein or adenosylethionine synthesis. No mutant alteration in accumulation of ethionine from the medium was detected. In the presence of ethionine, the parental strain accumulated 25 times more adenosylethionine than did the mutant. However, with methionine, only twice as much adenosylmethionine was accumulated by the parental strain as by the mutant.

Berry, L. Joe (Bryn Mawr College, Bryn Mawr, Pa.), and Dorothy S. Smythe. Some metabolic aspects of tolerance to bacterial endotoxin. J. Bacteriol. 90:970-977. 1965.-The tolerance to bacterial endotoxins which is produced in mice given a series of daily injections of heat-killed Salmonella typhimurium failed to occur when actinomycin D was administered with the heat-killed cells. Neither ethionine nor 2-thiouracil, when given with endotoxin, altered the development of tolerance. An injection of endotoxin, actinomycin D, or ethionine lowered the activity of the liver enzyme tryptophan pyrrolase more significantly at either 4 or 17 hr postinjection in normal mice than in tolerant mice. Similarly, an injection of either saccharated iron oxide or Thorotrast lowered liver tryptophan pyrrolase activity more extensively in normal than in tolerant animals. Activation of the reticuloendothelial system (RES) of tolerant mice, as determined by an accelerated rate of carbon clearance from the blood, was observed, but this was prevented by the appropriate dose of actinomycin D. Similar results were obtained when saccharated iron oxide, rather than endotoxin, was used to activate the RES, but these animals were not resistant to endotoxin and their tryptophan pyrrolase was normally diminished after an injection of endotoxin. Thus, RES activation may occur without tolerance developing. A more nearly normal level of enzyme activity appears to be characteristic of the tolerant state.

The control of methionyl-transfer ribonucleic acid (tRNA) synthetase (l-methionine: soluble RNA ligase [adenosine monophosphate]) was studied in methionyl-tRNA synthetase mutants of Escherichia coli and Salmonella typhimurium. The results of activity determinations with crude extracts indicate that this enzyme of the E. coli mutant strain possessed a reduced affinity for methionine-tRNA, whereas this enzyme of the S. typhimurium mutant exhibited a decreased affinity for l-methionine. The differential rate of methionyl-tRNA synthetase formation in these two mutants was several-fold greater than that of the respective parental strains. On the other hand, the level of in vivo aminoacylation of methionine-tRNA was only about one-third that of the parent strains. These results suggest that aminoacylation of methionine-tRNA is a necessary step in repression control of methionyl-tRNA synthetase of both E. coli and S. typhimurium strains.

Assay of the first enzyme unique to methionine biosynthesis, homoserine-O-transsuccinylase, in metJ and metK regulatory mutants of Salmonella typhimurium showed that synthesis of the enzyme was derepressed seven- and fourfold, respectively. The possibility of noncoordinate regulation of the methionine enzymes is discussed. In metA feedback-resistant mutants, the enzyme activity can be inhibited in vitro by 10 mmS-adenosylmethionine but not by 10 mm l-methionine; hence, the synergistic inhibition found for the wild-type enzyme is not effective in these latter mutants.

The properties of a specific system for the transport of S-adenosylmethionine in yeast are described. The process was pH-, temperature-, and energy-dependent, and showed saturation kinetics. The K(m) for the system was 3.3 x 10(-6)m. Of the S-adenosylmethionine moieties tested, only S-adenosylhomocysteine competitively inhibited the uptake of the adenosylsulfonium compound. Adenine, adenosine, methionine, homocysteine, and the sulfonium compound S-methylmethionine were without effect. The analogue S-adenosylethionine showed competitive inhibition. Under conditions of inhibition of protein synthesis by cycloheximide or methionine starvation, permease activity was stable. The mutant sam-p3 apparently was able to transport S-adenosylmethionine only by diffusion. Uptake by diploids containing this mutation was directly proportional to the gene dose.

The effects of mutations occurring at three independent loci, eth2, eth3, and eth10, were studied on the basis of several criteria: level of resistance towards two methionine analogues (ethionine and selenomethionine), pool sizes of free methionine and S-adenosyl methionine (SAM) under different growth conditions, and susceptibility towards methionine-mediated repression and SAM-mediated repression of some enzymes involved in methionine biosynthesis (met group I enzymes). It was shown that: (i) the level of resistance towards both methionine analogues roughly correlates with the amount of methionine accumulated in the pool; (ii) the repressibility of met group I enzymes by exogenous methionine is either abolished or greatly lowered, depending upon the mutation studied; (iii) the repressibility of the same enzymes by exogenous SAM remains, in at least three mutants studied, close to that observed in a wild-type strain; (iv) the accumulation of SAM does not occur in the most extreme mutants either from endogenously overproduced or from exogenously supplied methionine: (v) the two methionine-activating enzymes, methionyl-transfer ribonucleic acid (tRNA) synthetase and methionine adenosyl transferase, do not seem modified in any of the mutants presented here; and (vi) the amount of tRNA(met) and its level of charging are alike in all strains. Thus, the three recessive mutations presented here affect methionine-mediated repression, both at the level of overall methionine biosynthesis which results in its accumulation in the pool, and at the level of the synthesis of met group I enzymes. The implications of these findings are discussed.

Ethionine supplementation of a defined medium for growth of Pseudomonas denitrificans inhibited vitamin B(12) overproduction and led to the elaboration of a red pigment. The pigment was shown to be coproporphyrin III. Inhibition by ethionine of cobalamin synthesis is probably due to interference of methylation of the corrin nucleus by methionine. Accumulation of coproporphyrin III is thought to result from interference by ethionine with the activity of methionine in the coproporphyrinogenase reaction; this would inhibit formation of heme, the feedback inhibitor and corepressor of delta-aminolevulinate synthetase, thus allowing unregulated synthesis of coproporphyrinogen III and its degradation product, coproporphyrin III. Betaine, known to be required for vitamin B12 overproduction, was found to be an essential requirement for porphyrin overproduction in the presence of ethionine. Low-level production of porphyrin, which occurs in the absence of ethionine, also required betaine supplementation. Betaine is thus required for overproduction of both corrins and porphyrins in P. denitrificans.