Specialized subregions of the bifunctional hisB gene of Salmonella typhimurium.

Forty-three hisB mutants of Salmonella typhimurium have been screened to determine the molecular size of the resulting histidinol phosphate phosphatase activity, one of the activities of a bifunctional enzyme produced by this gene which also controls imidazole glycerol phosphate dehydrase activity. Mutation in hisB can lead to the loss of both phosphatase and dehydrase activities, or only of dehydrase activity. Through the use of nonsense mutants lacking dehydrase activity, a distinct point of transition was detected near the middle of hisB at which a dramatic change occurs in the size of the phosphatase enzyme that is synthesized. A missense mutant with a lesion in this region has a high-molecular-weight enzyme which is eluted in the void volume of a Sephadex G-200 column. The enzyme from nonsense mutants near the transition point have molecular weights near 40,000. Even though the buffer conditions are designed to favor the stabilization of the high-molecular-weight form, some mutants have both high- and low-molecular-weight forms. The polypeptide chain specified by the operator proximal part of hisB is sufficient to allow the expression of phosphatase activity. The synthesis of substantially less than the complete product of hisB resulted in association into a form similar to the native enzyme which was found in the void volume of a Sephadex G-200 column.