A role for a pyridoxne derivative in the multivalent repression of the isoleucine and valine biosynthetic enzymes

Metabolite gene regulation of the L-arabinose operon in Escherichia coli with indoleacetic acid and other indole derivatives

The ability of indole derivatives to facilitate RNA polymerase transcription of the L-arabinose operon in Escherichia coli was shown to require the catabolite activator protein (CAP) as well as the araC gene product. Adenosine 3',5'-monophosphate (cAMP) was not obligatory for araBAD transcription when the cells were grown in the presence of 1 mM indole-3-acetic acid or in the presence of indole-3-acetamide, indole-3-propionic acid, indole-3-butyric acid, or 5-hydroxyindole-3-acetic acid. However, these indole derivatives were unable to circumvent the cAMP requirement for the induction of the lactose and the maltose operons. Catabolic repression occurred when glucose was added to cells grown in the presence of L-arabinose and 1 mM indoleacetic acid or 1 mM cAMP. This effect was reversed at higher concentrations of indoleacetic acid or cAMP. The induction and the catabolite repression phenomena were quantitated by measuring the differential rate of synthesis of L-arabinose isomerase (the araA gene product). These results indicated that indole metabolites from various living systems may regulate gene expression and may be involved in "metabolite gene regulation."

Dual autogenous regulatory role of threonine deaminase in Escherichia coli K-12

We describe the regulatory properties of two strains carrying either the ilvA624 or the ilvA625 mutations, located in the structural gene for threonine deaminase. Crude extracts of both these strains possess a threonine deaminase activity migrating on polyacrylamide gels, differently from the wild type enzyme. Growth studies demonstrate that these mutations do not cause a limitation of isoleucine biosynthesis, suggesting normal catalytic activity of deaminase. A regulatory consequence of the ilvA624 allele is a derepression of the isoleucine-valine biosynthetic enzymes, which is recessive to an ilvA+ allele. The ilvA625 mutation causes a derepression which is dominant in an ilvA625/ILVA+ diploid. We interpret these data assuming that threonine deaminase, previously shown to be an autogenous regulator of the ilv genes, lacks a repressor function in the ilvA624 mutant, while in the ilvA625 mutant it is a better activator than wild type threonine deaminase. The data are discussed in terms of a model requiring that threonine deaminase, or a precursor of it, is in equilibrium between two forms, one being an activator of gene expression and the other being a repressor.

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.

Deoxyribonucleic acid-directed in vitro synthesis of ilv-specific messenger ribonucleic acid by extracts of Escherichia coli K-12

The synthesis of ilv-specific messenger ribonucleic acid (mRNA) by extracts of Escherichia coli K-12 has been demonstrated in a deoxyribonucleic acid (DNA)-dependent, coupled transcription-translation system. ilv-Specific mRNA was determined by hybridization either to double-stranded lambdacI857St68h80dilv DNA (lambdah80dilv DNA) immobilized on nitrocellulose filters or to its separate l and r strands in liquid. During conditions optimal for protein synthesis, slightly more than 6% of the total [(3)H]RNA synthesized by S-30 extracts of the threonine deaminase-negative strain CU5136 was ilv-specific. Of this RNA, nearly 30% was complementary to the l (correct) strand. Total ilv-specific mRNA synthesis in vitro was not affected by omission of valine or all 20 amino acids from the reaction mixture. Hybridization of ilv-specific mRNA made in vitro to the l strand of lambdah80dilv DNA was effectively reduced in the presence of unlabeled RNA extracted from an ilv derepressed strain but not from an ilv deletion strain. In a purified transcription system, employing commercial RNA polymerase, twofold more ilv-specific mRNA was synthesized than in the coupled system, but this increase was entirely due to greater transcription of the r (incorrect) strand. An S-30 extract prepared from a strain isogenic to strain CU5136 but derepressed for ilvA gene expression synthesized twofold more ilv-specific mRNA in the coupled system. The significance of these findings is discussed.

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.

Complementation between different mutations in the ilvA gene of Escherichia coli K-12

An ilvA mutation carried by a ø80i(lambda)dilv transducing phage complemented some ilvA mutations and did not complement others. Complementation was accompanied by appearance of threonine deaminase activity in vivo. These results divided the ilvA mutations into two sets which formerly appeared to define two cistrons.

Derepression of synthesis of the aminoacyl-transfer ribonucleic acid synthetases for the branched-chain amino acids of Escherichia coli

The kinetics of derepression of valyl-, isoleucyl-, and leucyl-transfer ribonucleic acid (tRNA) synthetase formation was examined during valine-, isoleucine-, and leucine-limited growth. When valine was limiting growth, valyl-tRNA synthetase formation was maximally derepressed within 5 min, whereas the rates of synthesis of isoleucyl-, and leucyl-tRNA synthetases were unchanged. Isoleucine-restricted growth caused a maximal derepression of isoleucyl-tRNA synthetase formation in 5 min and derepression of valyl-tRNA synthetase formation in 15 min with no effect on leucyl-tRNA synthetase formation. When leucine was limiting growth, leucyl-tRNA synthetase formation was immediately derepressed, whereas valyl- and isoleucyl-tRNA synthetase formation was unaffected by manipulation of the leucine supply to the cells. These results support our previous findings that valyl-tRNA synthetase formation is subject to multivalent repression control by both isoleucine and valine. In contrast, repression control of iso-leucyl- and leucyl-tRNA synthetase formation is specifically mediated by the supply of the cognate amino acid.

Autogenous regulation of gene expression

A new term, autogenous regulation, is used to describe a phenomenon that is not a new discovery but rather is newly appreciated as a mechanism common to a number of systems in both prokaryotic and eukaryotic organisms. In this mechanism the product of a structural gene regulates expression of the operon in which that structural gene resides. In many (perhaps all) cases, the regulatory gene product has several functions, since it may act not only as a regulatory protein but also as an enzyme, structural protein, or antibody, for example. In a few cases, this protein is the multimeric allosteric enzyme that catalyzes the first step of a metabolic pathway, gearing together the two most important mechanisms for controlling the biosynthesis of metabolites in bacterial cells-feedback inhibition and repression. Autogenous regulation may provide a mechanism for amplification of gene expression (84); for severe and prolonged inactivation of gene expression (85); for buffering the response of structural genes to changes in the environment (45, 52); and for maintaining a constant intracellular concentration of a protein, independent of cell size or growth rate (86). Thus, autogenous regulation provides the cell with means for accomplishing a number of different regulatory tasks, each suited to better satisfying the needs of the organism for its survival.

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

In Escherichia coli, the three branched-chain amino acid activating enzymes appear to be essential for multivalent repression of the isoleucine- and valine-forming enzymes. The results of experiments with a mutant, strain CU18, having an altered threonine deaminase, indicate that free isoleucine and some form of threonine deaminase (the product of the ilvA gene) are also involved in multivalent repression. This strain exhibits abnormally high derepressibility but normal repressibility of its ilv gene products, and its threonine deaminase is inhibited only by high concentrations of isoleucine. In strain CU18, the isoleucine analogue, thiaisoleucine, is incapable of replacing isoleucine in the multivalent repression of the ilv genes, whereas the analogue can fully replace the natural amino acid in repression in other strains examined. The dipeptide, glycyl-leucine, which, like isoleucine, is a heterotropic negative effector of threonine deaminase but is not a substrate for isoleucyl-transfer ribonucleic acid synthetase, can completely prevent the accumulation of threonine deaminase-forming potential during isoleucine starvation in strains with normal threonine deaminases. It can not, however, prevent such accumulation in strain CU18 or in other strains in which threonine deaminase is insensitive to any concentration of isoleucine.

Autoregulation: a role for a biosynthetic enzyme in the control of gene expression

It was previously proposed, primarily on the basis of evidence in vitro, that L-threonine deaminase, the ilvA gene product, is required for repression of its own synthesis and for repression of the other genes in the ilv-ADE operon. In this communication, evidence in vivo is presented that supports this autoregulatory model. Further evidence is presented that suggests that L-threonine deaminase is also required for induction of the ilvC gene product. The autoregulatory model is presented in an expanded form to include recent evidence that L-threonine deaminase (EC 4.2.1.16) is a central element for repression of the ilvADE and ilvB operons, and for induction of the ilvC operon.