Translational and transcriptional components of repression by arginine in Escherichia coli

Evidence for translational repression of arginine biosynthetic enzymes in Escherichia coli: altered regulation in a streptomycin-resistant mutant

The formation and repressibility of the arginine biosyntietic enzymes acetylornithine delta-aminotransferase (EC 2.6.1.11), acetylornithine deacetylase (EC 3.5.1.16), ornithine carbamoyltransferase (EC 2.1.3.3), and argininosuccinate lyase (EC 4.3.2.1) were studied in an Escherichia coli W derivative (strain 250-10) that carries (a) a mutant allele of the argR regulatory gene causing a diminished repression-derepression range and (b) a streptomycin resistance mutation. In comparison with the streptomycin-sensitive parent 250, all four enzymes (a) are formed as smaller proportions of the total protein (overall range, 12% to 71%), whether the conditions are repressive (arginine excess) or derepressive (arginine restriction), and (b) show increased repressibility ratios, the carbamoyltransferase giving the largest increase (from 5.7 to 25.0). These effects appear to depend on the concurrent expression of the regulatory-gene and streptomycin resistance mutations, as indicated by analogous experiments with canavanine-resistant mutants of 250-10 that have partial argR- character. The results provide evidence for translational repression in the arginine system, and are interpreted in terms of a functional interaction of a mutant arginine repressor with a mutant S12 ribosomal protein. The locale of translational repression may be near the site of S12, and this mode of regulation may involve initiational selectivity of groupwise recognizable arginine messenger RNA's.

Effect of arginine on the stability and size of argECBH messenger ribonucleic acid in Escherichia coli

The chemical stability of argECBH messenger ribonucleic acid (mRNA) produced by Escherichia coli was found to be unaltered during steady-state repression by arginine. During extreme arginine deprivation, the increase in argECBH mRNA stability was related to general effects of amino acid starvation on mRNA stability. Thus a mechanism whereby argECBH gene expression is regulated by altering the decay rate of this mRNA is not consistent with our data. Sucrose gradient analysis followed by hybridization revealed that both heavy (14S) and light (8S) components of argECBH mRNA were produced by cells of E. coli K-12 grown without arginine, whereas predominantly light (8S) mRNA was produced by cells grown with arginine. A functional argR gene and the EC portion of the argECBH cluster were found essential for the arginine restriction of heavy-mRNA production. Experiments suggest that light argECBH mRNA did not arise from heavy message, and 8u% of both light and heavy mRNA was found bound to ribosomes. The data appear most consistent with the notion that a second site of control by arginine regulates the amounts of light and heavy arginine mRNA in the cell either by early termination of transcription or by endonucleolytic processing. Consideration of these data in conjunction with those of the accompanying report (Krzyzek and Rogers, 1976) permits the tentative conclusion that light argECBH mRNA is not translated into active enzymes and is thus responsible for the discrepancy between the high content of hybridizable mRNA and low rates of enzyme synthesis found during arginine repression.

Regulation of argE-argH expression with arginine derivatives in Escherichia coli: extreme non-uniformity of repression and conditional repressive action

In regulatory studies of the arginine biosynthetic system of Escherichia coli, alpha-N-acetyl-l-arginine (AcA) is a useful restrictive arginine source. In strain 39A-23R3 (argA(-)), at 25 mug/ml, AcA gives suboptimal growth rates and is fully derepressive for acetylornithinase (specified by argE) and approximately 50% derepressive for argininosuccinase (specified by argH). At 10 mug/ml, the growth rate decreases, whereas the extent of derepression is unchanged; at 500 mug/ml, full repression results. In strain 3670 (argB(-)argG(-)), AcA (25 mug/ml) leads to partial derepression of acetylornithinase but full repression of argininosuccinase. Thus, the repression patterns for both strains, although not identical, are nonuniform. AcA utilization is antagonized by alpha-N-acetyl-l-ornithine (AcO). In strain 3670 (blocked before and after acetylornithinase), the growth rate on AcA (25 mug/ml) is lowered by AcO (500 mug/ml); acetylornithinase is completely derepressed, whereas argininosuccinase is fully repressed. This difference in regulatory behavior represents extreme nonuniform repression. Unexpectedly, the effect of AcO is attributable to the conversion of AcO to citrulline (Cit). In strain 3670, mixtures of AcA (25 mug/ml) and Cit (300 mug/ml) permit complete derepression of acetylornithinase; there is evidence that Cit enters the cell. In contrast, in the arginine-limited chemostat, Cit represses acetylornithinase. These opposite regulatory effects of Cit appear to stem from the difference in arginine restriction. AcA enters the cell via AcO permease and is deacylated by acetylornithinase (K(m), 5.0 mM). AcA competitively inhibits AcO cleavage (K(i), 2.4 mM), but Cit is not inhibitory. The antagonism of AcA utilization by AcO or Cit is thought to be exerted at the AcO permease.