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

Abstract

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 tRNAH"S, 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.Extensive studies on the regulation of biosynthetic operons in microorganisms have shown that there are two different mechanisms by which end product biosynthesis is regulated. One mechanism, feedback inhibition (fine control), involves inhibition of the first enzyme of the pathway by the end product (43, 48) and determines the internal pool of this metabolite in the cell. The other mechanism, repression and derepression (coarse control), involves changes in the intracellular concentrations of the biosynthetic enzymes (16,44).The results of several studies suggest that feedback-sensitive first enzyme of a biosynthetic operon is involved in repression (5,6,13,20,21,24,26,36,40). In Salmonella typhimurium, the involvement of the first enzyme for histidine biosynthesis (G enzyme) [N-1-(5'-phosphoribosyl) adenosine triphosphate: pyrophosphate phosphoribosyltransferase, EC 2.4.2.171 in the regulation of the operon is well documented (20,21,36). In the present work, results of experiments designed to study derepression and repression of the histidine operon lead us to postulate that direct participation of the first enzyme in regulation at the level of translation involves the state of the feedback site of the enzyme. MATERIALS AND METHODS Bacterial strains. The organisms employed in this study, obtained from the collection of P. E. Hartman (12), were the LT2 (wild type) strain of S. typhimurium and several mutants derived from it (Table 1).Growth conditions and experimental design. Cells were routinely grown in minimal medium (45) with glucose at 0.4%. The cultures were aerated in a rotary shaker at 37 C. The histidine auxotrophs were grown in the presence of a sufficient amount of L-histidine to support growth to an absorbance at 650 nm of approximately 0.4; after depletion of histidine, growth was controlled with L-hi...

Structural and physiological studies of the Escherichia coli histidine operon inserted into plasmid vectors

“…The gene number, order, and orientation (26,29) have been found to coincide with those of the S. typhimurium operon (32). Regulatory mutants have been characterized, and they fall into the classes hisR, hisS, hisT, hisU, and hisO (16,23,43, and F. Blasi, unpublished data). Studies in S. typhimurium show that, with the exception of the hisO class (50), all his regulatory genes affect either the structure (51), the quantity, the maturation, or the charging of histidyl-tRNAH3 (39).…”