REGULATION OF HEME-BIOSYNTHESIS IN SALMONELLA-TYPHIMURIUM - ACTIVITY OF GLUTAMYL-TRANSFER-RNA REDUCTASE (HEMA) IS GREATLY ELEVATED DURING HEME LIMITATION BY A MECHANISM WHICH INCREASES ABUNDANCE OF THE PROTEIN

catalyzes the first committed step in heme biosynthesis. We report tha t when heme limitation is imposed on cultures of S. typhimurium, gluta myl-tRNA reductase (HemA) enzyme activity is increased 10- to 25-fold. Heme limitation was achieved by a complete starvation for heme in hem B, hemE, and hemH mutants or during exponential growth of a hemL mutan t in the absence of heme supplementation. Equivalent results were obta ined by both methods. To determine the basis for this induction, we de veloped a panel of monoclonal antibodies reactive with HemA, which can detect the small amount of protein present in a wild-type strain. Wes tern blot (immunoblot) analysis with these antibodies reveals that the increase in HemA enzyme activity during heme limitation is mediated b y an increase in the abundance of the HemA protein. Increased HemA pro tein levels were also observed in heme-limited cells of a hemL mutant in two different E. coli backgrounds, suggesting that the observed reg ulation is conserved between E. coli and S. typhimurium. In S. typhimu rium, the increase in HemA enzyme and protein levels was accompanied b y a minimal (less than twofold) increase in the expression of hemA-lac operon fusions; thus HemA regulation is mediated either at a posttran scriptional step or through modulation of protein stability,