Role for free isoleucine of glycyl-leucine in the repression of threonine deaminase in Escherichia coli

Role of protein degradation in the survival of carbon-starved Escherichia coli and Salmonella typhimurium

When an Escherichia coli K-12 culture was starved for glucose, 50% of the cells lost viability in about 6 days. When a K-12 mutant lacking five distinct peptidase activities, CM89, was starved in the same manner, viability was lost much more rapidly; 50% of the cells lost viability in about 2 days, whereas a parent strain lacking only one peptidase activity lost 50% viability in about 4 days. Compared with the wild-type strain and with its parent strain CM17, CM89 was defective in both protein degradation and protein synthesis during carbon starvation. Similar results were obtained with glucose-starved Salmonella typhimurium LT2 and LT2-derived mutants lacking various peptidase activities. An S. typhimurium mutant lacking four peptidases, TN852, which was deficient in both protein degradation and synthesis during carbon starvation (Yen et al., J. Mol. Biol. 143:21-33, 1980), was roughly one-third as stable as the isogenic wild type. Isogenic S. typhimurium strains that lacked various combinations of three of four peptidases and that displayed protein degradation and synthesis rates intermediate between those of LT2 and TN852 (Yen et al., J. Mol. Biol. 143:21-33, 1980) displayed corresponding stabilities during carbon starvation. These results point to a role for protein degradation in the survival of bacteria during starvation for carbon.

Cloning of the ilvA538 gene coding for feedback-hypersensitive threonine deaminase from Escherichia coli K-12

A variety of experimental results implicate the ilvA gene product, threonine deaminase, as an autoregulatory protein that affects the expression of its own gene and those coding for some related proteins. Some of the most direct evidence comes from the analysis of mutations in the ilvA gene with pleiotropic genetic regulatory effects. The most extensively documented mutation, ilvA538, lowers the expression of and abolishes repression control of the ilvGEDA transcription unit. A pleiotropic effect of the ilvA538 mutation, which may be either incidental or mechanistically related to the loss of repression control, renders threonine deaminase feedback hypersensitive to the inhibition of catalytic activity by the pathway end product, isoleucine. We transferred this mutation to lambda dilv phage and pBR322 derivatives. Direct enzyme assay of the plasmid- and phage-coded ilvA538 gene product in delta ilv hosts confirmed the feedback hypersensitivity of the enzyme product. In conjunction with the ilvG671 (phenotype, ILvG+ Valr; previously designated ilvO671) allele located in cis, high levels of the plasmid and lambda dilv phage-coded mutant enzyme suitable for protein purification were observed. Deletion mapping experiments with lambda dilv phage confirmed that the ilvA538 mutation, and not mutations promoter proximal to ilvD (transcription is from ilvG to ilvA), confer a loss of repression control. These genetic mapping studies indicate, however, that an additional mutation(s) may be present that contributes, at least in part, to the reduced enzyme levels in strains with the ilvA538 mutation.

Threonine deaminase from Escherichia coli: feedback-hypersensitive enzyme from a genetic regulatory mutant

A mutation, ilvA538, in the gene coding for the biosynthetic L-threonine deaminase of Escherichia coli K-12 has previously been demonstrated to have pleiotropic regulatory effects leading to low and invariant expression of some of the isoleucine-valine biosynthetic enzyme, and altered expression of the branched-chain aminoacyl-tRNA synthetases. Strain PS187, which carries the ilvA538 allele, has a partial growth requirement for L-isoleucine and is characterized by a sensitivity to growth inhibition by L-leucine. The experiments reported here demonstrate that the L-threonine deaminase produced by strain PS187 is hypersensitive to inhibition by the pathway end product L-isoleucine. In addition, L-leucine, which acts at relatively high concentrations in vitro as an inhibitor of L-threonine deaminase from the wild type, is a more potent inhibitor of the activity of the mutant enzyme. Forty-six derivatives of strain PS187 were isolated as spontaneous mutants resistant to the growth-inhibitory effects of L-leucine. Two of these, strains MSR14 and MSR16, produce an L-threonine deaminase that is more resistant than the wild type to L-isoleucine inhibition, and intermediate between the wild type and strain PS187 with respect to L-leucine inhibition. Strains MSR14 and MSR16 produce L-threonine deaminase and dihydroxyacid dehydrase, the ilvD gene product, at the low levels characteristic of the parent strain. Other L-leucine-resistant derivatives of strain PS187 produce higher levels of the feedback-hypersensitive L-threonine deaminase. Thus, the sensitivity to growth inhibition by L-leucine observed with strain PS187 appears to be related both to the hypersensitivity of L-threonine deaminase to inhibition of catalytic activity and to the low level of ilv gene expression. The results reported here indicated that L-threonine deaminase is structurally altered in strain PS187, and thus provide further support for the proposal that L-threonine deaminase participates as a genetic regulatory element for the expression of the branched-chain amino acid biosynthetic enzymes.

Expression of a valine-resistant acetolactate synthase activity mediated by the ilv O and ilv G genes of Escherichia coli K-12

A strain carrying the ilv0603 mutation has been isolated in E. coli K-12 and its characteristics were found to be very similar to those previously reported by Ramakrishnan and Adelberg (1965a) for other ilv0 mutants. The strain carrying the ilv0603 mutation is resistant to valine inhibition (Valr) and we show that this resistance depends on the expression of a newly recognized gene, ilvG, which is located at min 75, between ilvE and ilvD on the E. coli K-12 map. The ilvG gene causes the expression of a Valr acetolactate synthase, which is detectable only when the ilv0603 mutation is also present in cis on the same chromosome. Under these conditions the Valr acetolactate synthase activity is eluted, on a hydroxylapatite column, at an ionic strength slightly lower than that required for elution of the remaining acetolactate synthase activity (sensitive to valine inhibition). The Valr peak is missing in a strain carrying an ilvG (amber) mutation.

Derepression and repression of the histidine operon: role of the feedback site of the first enzyme

Thiazolealanine, a false feedback inhibitor, causes transient repression of the his operon previously derepressed by a severe histidine limitation in strains with a wild-type or feedback-hypersensitive first enzyme but not in feedback-resistant mutants. Since experiments reported here clearly demonstrate that thiazolealanine is not transferred to tRNAHis, it is proposed that this "transient repression" is effected through the interaction of thiazolealanine with the feedback site of the enzyme. Experiments in the presence of rifampin indicate that this thiazolealanine-mediated effect is exerted at the level of translation. We conclude that histidine (free), in addition to forming co-repressor, also represses the operon at the level of translation through feedback interaction with the first enzyme of the pathway (adenosine 5'-triphosphate phosphoribosyltransferase). Rates of derepression in feedback-resistant strains are roughly half of those observed in controls, suggesting a positive role played by a first enzyme with a normal but unoccupied feedback site. Some feedback-resistant mutants, in contrast to the wild type, were unable to exhibit derepression under histidine limitation caused by aminotriazole.

Inhibition of Escherichia coli isoleucine biosynthesis by isoleucine tetrazole

Growth of a derivative of Escherichia coli K-10 was strongly inhibited by 2 times 10(-4) M L-5(1-amino-2-methylbutyl)-tetrazole (isoleucine tetrazole). Growth inhibition was reversed by isoleucine, threonine, glycyl-L-isoleucine, or glycyl-L-threonine, and, in a valine-resistant mutant, by L-valine. Partial reversal of growth inhibiton was effected by L-leucine, L-methionine, or L-homoserine. The tetrazole inhibited the activity of the biosynthetic threonine deaminase (EC 4.2.1.16 L-threonine hydrolyase [deaminating]), the inhibition being relieved by L-valine. The tetrazole also inhibited isoleucyl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.5 L-isoleucine: tRNA ligase [adenosine monophosphate]), but was without effect on the activities of alpha-isopropylmalate synthetase or acetohydroxy acid synthetase. One class of isoleucine tetrazole-resistant mutants produced biosynthetic threonine deaminases which were no longer subject to feedback inhibition by either isoleucine or the tetrazole.

Synthesis of branced-chain aminoacyl-transfer ribonucleid acid synthetases in a Salmonella typhimurium mutant with an altered biosynthetic L-threonine deaminase

The differential rates of synthesis of the three branched-chain aminoacyl-transfer ribonucleic acid synthetases were measured in Salmonella typhimurium LT-2 and a mutant, ilvA504. The mutant produced an l-threonine deaminase with a decreased affinity for its cofactor, pyridoxal-5'-monophosphate. The addition of pyridoxal-5'-monophosphate to cultures of strain ilvA504 growing in excess isoleucine, valine, and leucine resulted in an increased rate of growth and repression of the synthesis of the isoleucine and valine biosynthetic enzymes. No differences in the rate of synthesis of the branched-chain aminoacyl-transfer ribonucleic acid synthetases were observed in cultures of ilvA504 growing with or without added pyridoxal-5'-monophosphate. The differential rates of synthesis of all three enzymes were similar to the rates measured in strain LT-2. These experiments suggest that different forms of the ilvA gene product are involved in the regulation of the branched-chain amino acid biosynthetic enzymes and the branched-chain aminoacyl-transfer ribonucleic acid synthetases.