Relationship between methionyl transfer ribonucleic acid cellular content and synthesis of methionine enzymes in Saccharomyces cerevisiae

Dynamic Model of the Process of Protein Synthesis in Eukaryotic Cells

Protein synthesis is the final step of gene expression in all cells. In order to understand the regulation of this process, it is important to have an accurate model that incorporates the regulatory steps. The model presented in this paper is composed of set of differential equations which describe the dynamics of the initiation process and its control, as well as peptide elongation, starting with the amino acids available for peptide creation. A novel approach for modeling the elongation process permits useful prediction of protein production and consumption of energy and amino acids, as well as ribosome loading rate and ribosome spacing on the mRNA.

Studies on the regulation of the branched chain amino acyl-tRNA synthetases of the fungusNeurospora crassa

The specific activities of the branched chain amino acyl-tRNA synthetases from the cytosolic and mitochondrial fractions ofN. crassa were low in dormant conidia and increased during germination, reaching a maximum 8 h after inoculation. This stage of development is characterised by high rates of many other cellular activities.The increases in activity of synthetases of both cytosol and mitochondria are inhibited by cycloheximide indicating that they are synthesized on cytoplasmic ribosomes. The mitochondrial synthetases show a stimulation of their specific activity when mitochondrial RNA and protein synthesis are inhibited by either ethidium bromide or chloramphenicol suggesting that a mitochondrial translation product regulates the synthesis of the mitochondrial synthetases.The activities of amino acyl-tRNA synthetases are dependent on energy production. When respiration is uncoupled from oxidative phosphorylation, synthetase specific activities decrease although the activities of other mitochondrial enzymes like NADH-dehydrogenase increase. This phenomenon suggests that more than one mechanism regulates the synthesis of mitochondrial proteins which are formed on cytoplasmic ribosomes.The synthesis of branched chain amino acyl-tRNA synthetases ofNeurospora is neither repressed by their cognate amino acids, nor is there inhibition by the precursors of these amino acids, as has been observed in other amino acyl-tRNA synthetases of various organism includingNeurospora.

Further studies of the action of methionyl adenylate on chick embryo fibroblasts

Methionyl adenylate (Met-AMP) inhibits protein synthesis by interacting with methionyl-tRNA synthetase. Addition of 1--3 mM inhibitor to chick embryo fibroblasts rapidly stops protein synthesis and DNA synthesis but not RNA synthesis. These effects can be reversed by renewal of the medium. The extent and reversibility of protein and DNA syntheses depend on the concentration of MetAMP in the cultures, the length of exposure and the cellular density. MetAMP is recognised by several enzymes as substrate and/or as inhibitor. MetAmp is degraded to methionol plus 5'-adenylic acid by 5'-phosphodiesterase. Adenosine deaminase, adenylic acid deaminase and 3':5'-phosphodiesterase cannot use MetAMP as substrate but the last enzyme is inhibited. The presence of MetAMP in cultures provokes a small but reproducible increase in the level of methionyl-tRNA synthetase and 5'-phosphodiesterase.

Genetics of borrelidin resistant mutants of Saccharomyces cerivisiae and properties of their threonyl-tRNA-synthetase

Twenty-two borrelidin resistant mutants of Saccharomyces cerivisiae were isolated, studied genetically and their threonyl-tRNA-synthetase was investigated. The borrelidin resistant mutants are classified into four groups. In the first group borrelidin resistance is coupled to the gene HOM3 coding for aspartokinase, in the second group to the gene LEU1. The borrelidin resistance in group three and four is not coupled to anyone of the genetic markers tested. Borrelidin resistance exhibited dominant behavior in all mutants except in the mutant of group 4. The properties of the ThrRS of the mutants of group one, two and four was found to be like the ones of the wild types. However the mutants of group three exhibit a structurally altered ThrRS, which is no longer inhibited by borrelidin.

Studies of the regulation and reaction mechanism of the carbamyl phosphate synthetase and aspartate transcarbamylase of bakers' yeast

Kinetic studies of the carbamyl phosphate synthetase activity (CPSase) of bakers' yeast revealed an absolute requirement for K+ ions ; KM values for two of the substrates, glutamine and bicarbonate, were found to be 5 X 10(-4) M and 3 X 10(-3) M respectively. CPSase activity of the purified enzyme aggregate (M.W. 800,000) was extremely sensitive to UTP with a Ki of 2.4 X 10(-4) M. The purine nucleotide intermediate, XMP, was a strong activator of CPSase, acting at a site different from the regulatory site at which UTP binds ; XMP activation diminished at high concentrations of the substrate Mg-ATP. Studies of the reaction mechanism of CPSase revealed that it involved the sequential addition of the substrates bicarbonate and Mg-ATP, liberation of ADP, addition of glutamine, binding of ATP and then release of ADP and the product carbamyl phosphate. Studies of the reaction mechanism of the aspartate transcarbamylase (ATCase) of the aggregate yielded data which were not compatible with any of the usual models ; whichever reaction mechanism is ultivately found to fit the data, it will probably prove applicable both to the ATCase of the aggregate and to the disaggregated ATCase subunit (MW 138,000).

Methionine-and S-adenosyl methionine-mediated repression in a methionyl-transfer ribonucleic-acid synthetase mutant of Saccharomyces cerevisiae

A Saccharomyces cerevisiae mutant strain unable to grow at 38 C and bearing a modified methionyl-transfer ribonucleic acid (tRNA) synthetase has been studied. It has been shown that, in this mutant, the percentage of tRNAmet charged in vivo paralleled the degree of repressibility of methionine biosynthetic enzymes by exogenous methionine. On the contrary, the repression mediated by exogenous S-adenosylmethionine does not correlate with complete acylation of tRNAmet. Althought McLaughlin and Hartwell reported previously that the thermosensitivity and the defect in the methionyl-tRNA synthetase were due to the same genetic lesion (1969), no diffenence could be found in the methionyl-tRNA synthetase activity or in the pattern of repressibility of methionine biosynthetic pathway after growth at the premissive and at a semipermissive temperature. It appears that the mutant also exhibits some other modified characters that render unlikely the existence of only one genetic lesion in this strain. A genetic study of this mutant was undertaken which led to the conclusion that the thermosensitivity and the other defects are not related to the methionyl-tRNA synthetase modification. It was shown that the modified repressibility of methionine biosynthetic enzymes by methionine and the lack of acylation of tRNAmet in vivo follow the methionyl-tRNA synthetase modification. These results are in favor of the idea that methionyl-tRNAmet, more likely than methionine, is implicated in the regulation of the biosynthesis of methionine.

Effects of changes in growth rate on the levels of several aminoacyl-tRNA synthetases in yeast

When the growth rate of yeast cells is decreased (for instance by transferring the cells from a rich medium to a poor one, or when the cells enter the stationary phase, or when growth is inhibited by cycloheximide, or when valine is removed from the medium supporting growth of a valine-requiring mutant, or when a thermosensitive mutant is shifted to the non-permissive temperature) there is a decrease in the levels of the four aminoacyl-tRNA synthetases tested. Conversely, an increase in the growth rate is accompanied by an increase in the levels of the four enzymes. But when the growth rate is slowed down by decreasing the temperature of the medium, no effect on the levels of the aminoacyl-tRNA synthetases is observed. These results are consistent with the concept of "metabolic regulation" proposed by Parker and neidhardt.

Existence of two levels of repression in the biosynthesis of methionine in Saccharomyces cerevisiae: effect of lomofungin on enzyme synthesis

Derepression of a methionine biosynthetic enzyme (homocysteine synthase) has been studied after repression either by exogenous methionine or by exogenous S-adenosylmethionine (SAM). Lomofungin, which inhibits the synthesis of ribosomal precursor and messenger ribonucleic acid but not of protein in Saccharomyces cerevisiae, has been used in this system. It has been shown that the addition of this antibiotic prevents the derepression of homocysteine synthase after repression by exogenous methionine but not after repression by exogenous SAM. These experiments with lomofungin and the kinetics of repression after addition of methionine or SAM to the growth medium provide evidence that the repression induced by exogenous methionine acts at the transcriptional level whereas the repression induced by exogenous SAM acts at the translational level.

Biochemical and regulatory effects of methionine analogues in Saccharomyces cerevisiae

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.

tRNAs undermethylation in a met-regulatory mutant of Saccharomyces cerevisiae

A study of in vivo and in vitro methylation of tRNAs in regulatory mutants affected in methionine-mediated repression (eth2, eth3, eth10) has led to the following results: 1) The eth2-2 carrying strain presents a great undermethylation of its tRNAs of the same order of magnitude as observed during methionine starvation of methionine auxotrophs. 2) This undermethylation leads to a shift of the tRNAIII met peak on a BD cellulose column, while tRNAIII met peak is unchanged. 3) The study of a double mutant strain carrying eth2 and met2 mutations has shown that this undermethylation is a consequence of the high internal pool of methionine. 4) Undermethylation unequally affects the different bases and the different tRNA species.

Methionine-dependent synthesis of ribosomal ribonucleic acid during sporulation and vegetative growth of Saccharomyces cerevisiae

Methionine limitation during growth and sporulation of a methionine-requiring diploid of Saccharomyces cerevisiae causes two significant changes in the normal synthesis of ribonucleic acid (RNA). First, whereas 18S ribosomal RNA is produced, there is no significant accumulation of either 26S ribosomal RNA or 5.8S RNA. The effect of methionine on the accumulation of these RNA species occurs after the formation of a common 35S precursor molecule which is still observed in the absence of methionine. During sporulation, diploid strains of S. cerevisiae produce a stable, virtually unmethylated 20S RNA which has previously been shown to be largely homologous to methylated 18S ribosomal RNA. The appearance of this species is not affected by the presence or absence of methionine from sporulation medium. However, when exponentially growing vegetative cells are starved for methionine, unmethylated 20S RNA is found. The 20S RNA, which had previously been observed only in cells undergoing sporulation, accumulates at the same time as a methylated 18S RNA. These effects on ribosomal RNA synthesis are specific for methionine limitation, and are not observed if protein synthesis is inhibited by cycloheximide or if cells are starved for a carbon source or for another amino acid. The phenomena are not marker specific as analogous results have been obtained for both a methionine-requiring diploid homozygous for met13 and a diploid homozygous for met2. The results demonstrate that methylation of ribosomal RNA or other methionine-dependent events plays a critical role in the recognition and processing of ribosomal precursor RNA to the final mature species.

Effect of L-methioninyl adenylate on the level of aminoacylation in vivo of tRNA(Met) from Escherichia coli K12

In cells of E.coli K12 grown exponentially in minimal medium, tRNA(met), tRNA(leu) and tRNA(ile) are aminoacylated at 100%, 80% and 64%, respectively. On addition of L-methioninyl adenylate to the growth medium, one observes a specific deacylation of tRNA(met). When more than 35% of tRNA(met) is deacylated, growth rate is reduced and becomes proportional to the amount of methionyl-tRNA formed.

Effects of regulatory mutations upon methionine biosynthesis in Saccharomyces cerevisiae: loci eth2-eth3-eth10

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.

S-adenosyl methionine-mediated repression of methionine biosynthetic enzymes in Saccharomyces cerevisiae

S-adenosylmethionine (SAM) has been shown to provoke repression of some methionine-specific enzymes in wild-type cells, namely, adenosine triphosphate sulfurylase, sulfite reductase, and homocysteine synthetase. Repressive effects observed in SAM-supplemented cultures should be due to SAM per se, since the intracellular pool of SAM increases while the intracellular pool of methionine remains low and constant. Derepression brought about by methionine limitation is accompanied by a severe decrease in SAM as well as methionine pool sizes, although methionine adenosyl transferase is slightly derepressed. Different hypotheses have been considered to account for the previously reported implication of methionyl transfer ribonucleic acid and the presently reported SAM effects in this regulatory process.