ANAEROBIOSIS, FORMATE, NITRATE, AND PYRA ARE INVOLVED IN THE REGULATION OF FORMATE HYDROGENLYASE IN SALMONELLA-TYPHIMURIUM

Three groups of mutants defective in the fermentative production of gas were isolated from S. typhimurium LT2 subjected to transposition mutagenesis with phage Mu d(Apr lac). One group consisted of strains which lacked hydrogenase. The mutation site for this group was located in the vicinity of the known hyd gene. A 2nd group consisted of mutants which lacked the formate dehydrogenase associated with hydrogenase. The mutation site was located in 4 of them. It was not in the vicinity of the previously described fhlD gene but was instead located at 93 min on the Salmonella map. The 3rd mutant group, which consisted of strains that produced gas in triple sugar iron agar but not in nutrient agar supplemented with glucose, appeared to be pyrA mutants. The insertion site was located in the vicinity of pyrA, and they required arginine and pyrimidines for growth. Expression of the lac operon in the hyd mutants was induced by anaerobiosis. It was only slightly increased by the addition of formate under anaerobic conditions and slightly decreased by the addition of nitrate. Nitrate had no effect in an hyd::Mu d strain that also carried a chlC::Tn10 insertion. Full expression of the lac operon in the fhl mutants required both formate and anaerobic conditions. The presence of nitrate in addition to formate resulted in activities about half those obtained in its absence, even in the fhl::Mu d chlC::Tn10 double mutant. In the absence of formate, nitrate reduced expression only in the fhl::Mu d single mutants. Expression of the lac operon among the pyrA mutants was repressed by arginine and cytosine and also by anaerobiosis. An explanation for the involvement of pyrA in aerobic and anaerobic metabolism is proposed.