Participation of branched-chain amino acid analogues in multivalent repression

Control of isoleucine-valine biosynthesis in a valine-resistant mutant of Escherichia coli K-12 that simultaneously acquired azaleucine-resistance

A mutant of Escherichia coli K-12 isolated as being growth resistant to L-valine (Valr) was shown also to exhibit growth resistance to 4-azaleucine (Azlr). Transductional analysis indicated that Azlr is cotransduced with Valr at a frequency of 100% and both are linked to leu, ara, and carA. This mutation conferring valine and azaleucine growth resistance resulted in increased levels of isoleucine and valine biosynthetic enzymes as well as those of valyl- and isoleucyl-tRNA synthetases during growth in minimal and enriched media. Acquisition of Vals/Azls results in the restoration of normal regulation of both classes of ilv enzymes and normal patterns of the tRNA Ile species. The overall regulatory patterns observed for individual isoleucine and valine gene products suggest differential participation of isoleucine and valine and/or isoleucyl- and valyl-tRNA's in control of expression of the respective structural genes.

Threonine deaminase: autogenous regulator of the ilv genes in Escherichia coli K-12

In this paper we analyze the effect of mutations in three genes, ilvO, ilvA and rho, on the expression of the ilvEJGDA gene cluster of Escherichia coli K-12. The ilvO603 mutation causes a cis-dominant derepression of the ilvEJGDA genes. In particular, the ilvG gene, not expressed in the wild type, becomes expressed in the ilvO603 strain. We have introduced ilvA mutations (ilvA454 or ilvA628) in the ilvO603 strain and we show that ilvG expression requires the presence in cis of both an ilvO603 mutation and of an ilvA+ allele. The ilvG gene is not expressed when in trans is present an ilvO+, ilvA+ genotype. However, it is expressed when the chromosome in trans is ilvO603, ilvA+ (ilvG-). We suggest that ilvO603 is part of ilvA, the structural gene for threonine deaminase, and that threonine deaminase from the ilvO603 mutant binds the ilvO603 site and not the ilvO+ site. Therefore, the ilvA gene product would be a cis-acting protein. Mutations in the rho gene cause derepression of the ilvEJGDA gene cluster without a concomitant expression of the ilvG gene. We show that introduction of either a rho-218 or a rho-115 mutation into the ilvO603, ilvA454 double mutant causes expression of ilvG. We therefore suggest that the ilvA gene product, threonine deaminase, is involved in termination of transcription as an antagonist of the rho gene product. Introduction of ilvA454 into an ilvO603 strain causes also a decrease in expression of the ilvE, ilvJ and ilvD genes. This effect is maximum in the case of the ilvD gene and we studied it in detail in isogenic strains containing also the rho-218 mutation.

Transport of L-4-azaleucine in Escherichia coli

The uptake of L-4-azaleucine was examined in Escherichia coli K-12 strains to determine the systems that serve for its accumulation. L-4=Azaleucine in radio-labeled form was synthesized and resolved by the action of hog kidney N-acylamino-acid amidohydrolase (EC 3.5.1.B) on the racemic alpha-N-acetyl derivative of DL-[dimethyl-14C]4-azaleucine. L-4-Azaleucine is taken up in E. coli by energy-dependent processes that are sensitive to changes in the pH and to inhibition by leucine and the aromatic amino acids. Although a single set of kinetic parameters was obtained by kinetic experiments, other evidence indicates that transport systems for both the aromatic and the branched-chain amino acids serve for azaleucine. Azaleucine uptake in strain EO317, with a mutation leading to derepression and constitutive expression of branched-chain amino acid (LIV) transport and binding proteins, was not repressed by growth with leucine as it was in parental strain EO300. Lesions in the aromatic amino acid transport system, aroP, also led to changes in the regulation of azaleucine uptake activity when cells were grown on phenylalanine. Experiments on the specificity of azaleucine uptake and exchange experiments with leucine and phenylalanine support the hypothesis that both LIV and aroP systems transport azaleucine. The ability of external azaleucine to exchange rapidly with intracellular leucine may be an important contributor to azaleucine toxicity. We conclude from these and other studies that at least four other process may affect azaleucine sensitivity: the level of branched-chain amino acid biosynthetic enzymes; the level of leucine, isoleucine, and valine transport systems; the level of the aromatic amino acid, aroP, uptake system; and, possibly, the ability of the cell to racemize D and L amino acids. The relative importance of these processes in azaleucine sensitivity under various conditions is not known precisely.

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.