Properties of the imidazolylacetolphosphate aminotransferase produced in a mutant demonstrating no apparent genetic involvement of the structural gene.

Genetic studies with strain hisBH22 of Salmonella typhimurium indicate it contains a deletion within the histidine operon involving part of the hisH gene and all of the hisB gene, but not extending into the adjacent hisC gene which is adjacent to hisB. However, the specific activity of the hisC product, imidazolylacetolphosphate aminotransferase (EC 2.6.1.9), in this strain is only 10 to 15% of that found in extracts from other mutants with a normal hisC gene. We have examined the rate of aminotransferase synthesis in this mutant and we find that the rate of synthesis of aminotransferase activity is low in mutant hisBH22, but the rate increases as the temperature of growth is lowered from 37 to 23 C. The low rate of enzyme accumulation is not due to holoenzyme instability at 37 C but instead is due to apoenzyme instability at this temperature. By transducing the hisBH22 marker into a pyridoxine auxotroph and derepressing the histidine operon under conditions where the intracellular concentration of pyridoxal phosphate would be expected to be low, we were able to demonstrate significant apoenzyme production only at the lower temperature. We suggest that the explanation for low aminotransferase specific activity at 37 C is due to the presence of reduced numbers of catalytically active units caused by normal production of an unstable mutant apoenzyme with only approximately 15% of the molecules being activated to holoenzyme. The holoenzyme from strain hisBH22 is stable during growth of this strain at 37 C.