Transcriptional control of the isoleucine-valine messenger RNA's in E. coli K-12

Transcriptional polarity enhances the contribution of the internal promoter, ilvEp, in the expression of the ilvGMEDA operon in wild-type Escherichia coli K12

The ilvG gene of Escherichia coli K12 produces a cryptic peptide as a result of a frameshift mutation located approximately halfway through the coding sequence of the gene. This mutation is polar on expression of the downstream genes (ilvEDA) because transcription terminates within the translationally barren region that results from the mutation. Contrary to this, Salmonella typhimurium produces a full-length functional ilvG protein and is therefore unlikely to manifest this polarity event. E. coli K12 strains with mutations either in the ilvG gene (which restores a full-length protein) or in the rho gene, relieve this polarity suggesting that this event couples transcription and translation in a manner analogous to attenuation. This paper describes experiments designed to determine the molecular nature and location of the polarity event. Most significantly, this work establishes the contribution of the internal promoter (ilvEp, located downstream of the polar site) to the expression of the downstream genes in E. coli K12 wild-type and mutant strains (ilvG) and by extension to the role of this promoter in S. typhimurium. This analysis suggests that ilvEp contributes as much as 90% of ilvEDA expression in wild-type E. coli K12 and only 15% in wild-type S. typhimurium when grown under non-repressing conditions.

Cloning and expression of the ilvB gene of Escherichia coli K-12

A plasmid containing the ilvB operon, which codes for acetohydroxy acid synthase I of Escherichia coli K-12, was isolated using a ligated mixture of DNA from plasmid pBR322 and FilvB4 treated with endonuclease SalI. A shortened derivative of this plasmid was isolated by cloning a 3.4 kb bacterial fragment into plasmid pKEN005 to yield plasmid pTCN12. The orientation of the ilvB operon relative to plasmid genes was determined by restriction enzyme mapping. Measurement of the level of the product of the ilvB gene, acetohydroxy acid synthase I, indicated that plasmid pTCN12 contained a functional ilvB promoter and control region. The DNA frm this plasmid was used as a probe to show that the rate of synthesis of ilvB mRNA was proportional to the levels of acetohydroxy acid synthase I.

Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in acetohydroxyacid synthase I function in vivo

Mutations in Escherichia coli genes cpxA and cpxB together cause a temperature-sensitive defect in isoleucine and valine syntheses that is related specifically to acetohydroxyacid synthase I. This enzyme catalyzes the first pair of homologous reactions required for the synthesis of these two amino acids. At both permissive and nonpermissive temperatures, mutant cells containing ilvB (the structural gene for acetohydroxyacid synthase I) cloned in a derivative of plasmid pBR322 synthesized comparable amounts of ilvB mRNA and contained several times the enzyme activity normally required to sustain exponential growth, yet these cells remained temperature sensitive for growth in the absence of isoleucine and valine. These observations suggest that the primary effect of the cpx mutations is to block enzyme function in vivo. The enzyme was unstable in mutant cells at growth temperatures above 37 degrees C, but this instability appeared to be a secondary effect on the cpx mutations.

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.

Detection of messenger RNA from the isoleucine--valine operons of Salmonella typhimurium by heterologous DNA-RNA hybridization: involvement of transfer RNA in transcriptional repression

A hybridization assay using Escherichia coli K-12 DNA isolated from the specialized transducing bacteriophage gammaCI857St68h80 dilv was used to examine the rate of synthesis of the messenger RNA's (mRNA) derived from the isoleucine-valine (ilv) gene cluster of Salmonella typhimurium. In all cases examined, changes in ilv enzyme levels could be correlated with changes in the rate of synthesis of ilv mRNA. Several well characterized regulatory mutants of S. typhimurium had rates of synthesis of ilv mRNA 3 to 8-fold higher than the repressed wild-type strain. The increased rates of ilv mRNA synthesis found in a hisT strain as well as in isoleucyl-and leucyl-tRNA SYNTHETASE MUTANTS, STRONGLY SUGGESTS A ROLE FOR BRANCHED-CHAIN AMINOACYL-TRNA's in transcriptional control.