Interaction between the first enzyme for histidine biosynthesis and histidyl transfer ribonucleic acid

Previous studies suggested that phosphoribosyltransferase, which catalyzes the first step of the pathway for histidine biosynthesis in Salmonella typhimurium and which is sensitive to inhibition by histidine, plays a role in repression of the histidine operon. Recently, we showed that the enzyme has a high affinity for histidyl transfer ribonucleic acid (His-tRNA), which is known to participate in the repression process. In the present study, we have investigated further the interaction between the enzyme and His-tRNA. We found that His-tRNA binds at a site on phosphoribosyltransferase distinct from the catalytic site and the histidine-sensitive site; that the substrates of the enzyme inhibit the binding of His-tRNA, whereas histidine does not do so; that, once a complex has been formed between phosphoribosyltransferase and His-tRNA, the substrates of the enzyme decrease the stability of the complex, whereas histidine is without effect; and that purified phosphoribosyltransferase which has a defect in its inhibition by histidine (produced by mutation) displays an altered ability to bind His-tRNA, a finding which may be a reflection of the fact that mutants producing such a defective enzyme display an alteration of the repression process.