Active transport of exogenous S-adenosylmethionine and related compounds into cells and vacuoles of Saccharomyces cerevisiae

Saccharomyces cerevisiae 4094-B (alpha, ade-2, ura-1) in potassium phosphate buffer with glucose under aerobic conditions took up (-)S-adenosyl-l-methionine from the medium in sufficient quantity to permit the demonstration of its accumulation in the vacuole by ultraviolet micrography. The same result was obtained with (+/-)S-adenosyl-l-methionine, (+/-)S-adenosyl-d-methionine, and (-)S-adenosyl-l-ethionine. The rate of uptake was slow with (-)S-adenosyl-S(n-propyl)-l-homocysteine and S-adenosyl-d-homocysteine. S-Adenosyl-l-homocysteine was assimilated rapidly, but intracellular degradation precluded accumulation and ultraviolet micrographic studies. The uptake of 5'-methyl-, 5'-ethyl-, 5'-n-propylthioadenosine, and 5'-dimethylsulfonium adenosine was minimal.

Properties of P503 in Escherichia coli

The pigment (believed to be a tetrahydroporphyin) which appears, transiently, at 503 nm (P503) in the difference spectrum of intact cells of Escherichia coli B did not accumulate in cultures grown in medium supplemented with L-methionine. Accumulation of P503 was not prevented by structural analogues of L-methionine, nor by a mixture of natural amino acids excluding L-methionine. A methionine-requiring mutant of E. coli K-12 lacked P503 when grown in the presence of L-methionine, but P503 was present if D,L-ethionine was substituted for L-methionine in the growth medium. When L-methionine was added to air-oxidized cells which had been grown on minimal (glucose–salts) medium, the spectrum of P503 was produced specifically and rapidly. This effect of L-methionine in non-proliferating cells together with the inhibition by L-methionine of P503 accumulation in growing cultures may result from the stimulation of coproporphyrinogenase by methionine. The data indicate that P503 is an early intermediate in the conversion of coproporphyrinogen III to protoporphyrin IX rather than an intermediate in the porphyrin synthesis "by-pass' to coproporphyrin III.

Control of gene function in bacteriophage T4. IV. Post-transcriptional shutoff of expression of early genes

The selective and sequential shutoff of synthesis of early T4 proteins in bacteria infected with DNA-negative mutants is under the active control of one or more T4-induced proteins. Selective shutoff of synthesis of early T4 proteins is accompanied by a selective degradation of distinct species of T4 mRNA. We present circumstantial evidence that selective degradation of mRNA is the cause, and not the consequence, of selective termination of expression of early T4 genes. The mutation sp62 inactivates the shutoff mechanism and prevents the selective degradation of distinct species of T4 mRNA.

Excretion of enterochelin by exbA and exbB mutants of Escherichia coli

Escherichia coli mutants that are insensitive to colicins B and I hyperproduce and excrete the iron chelator enterochelin, which is an inhibitor of these colicins. These mutants are classified as exbA and exbB. The exbA mutants are chromium sensitive and require iron for growth, and the mutations are located in the tonB region at min 25 of the E. coli chromosome. tonB mutants in which the genome of phage lambda is inserted into the bacterial chromosome within the tonB gene also exhibit enterochelin excretion. The exbB mutants require methionine and probably result from deletions which are located between min 56 and 58. Colicin insensitivity, enterochelin excretion and methionine auxotrophy are recessive in exbB merodiploids. The methionine requirement of exbB strains is satisfied by cystathionine or homocysteine, and exbB mutants are sensitive to ethionine.

Illicit transport: the oligopeptide permease

The oligopeptide permease of Escherichia coli has been characterized by Payne, Gilvarg, and their colleagues. We have confirmed its existence in Salmonella typhimurium, and have isolated a series of mutants lacking the permease. We use this transport system for smuggling a histidine biosynthetic intermediate, histidinol phosphate ester, into the bacteria as its glycylglycyl derivative, Gly-Gly-histidinol phosphate. Free histidinol phosphate ester is not transported into Salmonella. Several amino-acid analogues are shown to be much more inhibitory to Salmonella when presented to the bacteria in the form of tripeptides than as the free amino acids. The implications of this work for practical purposes are discussed. The synthesis of Gly-Gly-histidinol phosphate is described.

Error theory and ageing in human diploid fibroblasts

Immunological and enzymatic assessments of lactate dehydrogenase in human lung fibroblasts strongly suggest that altered proteins accumulate in ageing cells. This supports but does not prove the error catastrophe theory of all death.

Methionine adenosyltransferase and ethionine resistance in Saccharomyces cerevisiae

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.

Studies of the ethylation of rat liver transfer ribonucleic acid after administration of L-ethionine

1. The ethylated nucleosides present in tRNA isolated from the livers of rats treated with 0.5g of l-ethionine/kg body wt. were investigated. Evidence that this tRNA contained N(2)-ethylguanine, N(2)N(2)-diethylguanine, N(2)-ethyl-N(2)-methylguanine, 7-ethylguanine, two ethylated pyrimidines and ethylated ribose groups was obtained. 2. Ethylation of bacterial tRNA was catalysed by extracts containing tRNA methylases prepared from rat liver by using S-adenosyl-l-ethionine as an ethyl donor, but the rate of ethylation was 20 times less than the rate of methylation with S-adenosyl-l-methionine as a methyl donor. 3. The principal product of such ethylation in vitro was N(2)-ethylguanine and traces of the other ethylated guanines and pyrimidines found in tRNA isolated from rats treated with ethionine in vivo were also found. 1-Ethyladenine was not formed, although 1-methyl-adenine is a major product of methylation of bacterial tRNA by these extracts, and 1-ethyladenine was not present in the rat liver tRNA isolated from ethionine-treated animals. 4. After injection of actinomycin D (15mg/kg body wt.) or l-methionine (1.0g/kg body wt.) before the ethionine, ethylation of tRNA was diminished by about 80% but not completely abolished. Administration of 1-aminocyclopentanecarboxylic acid (2.5g/kg body wt.) to inhibit the formation of S-adenosyl-l-ethionine inhibited ethylation of tRNA by 44%. 5. These results suggest that not all of the ethylation of tRNA that occurs in the livers of rats treated with ethionine is mediated by the action of tRNA methylases acting with S-adenosyl-l-ethionine as a substrate, but that this pathway does occur and accounts for a major part of the observed ethylation. 6. The results are discussed with reference to ethionine-induced hepatocarcinogenesis.

Metabolism of ethionine in ethionine-sensitive and ethionine-resistant cells of the enteric yeast Candida slooffii

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.