Differences between the anthranilate-5-phosphoribosylpyrophosphate phosphoribosyltransferases of Salmonella typhimurium strains LT2 and LT7.

The anthranilate-5-phosphoribosylpyrophosphate phosphoribosyltransferases (PRT), coded by the second structural gene (trpB) of the tryptophan (trp) operon in strains LT2 and LT7 of Salmonella typhimurium, differ from each other in a number of parameters. These include the apparent Km values for their substrates anthranilic acid and 5-phosphoribosylpyrophosphate, thermostability, sensitivity to substrate inhibition by anthranilic acid, as well as end-product inhibition by tryptophan and specific activity. The PRT of strain LT7 further differs from that of strain LT2 in that its apparent Km for 5-phosphoribosylpyrophosphate is three to seven times higher when associated with anthranilate synthase in the enzyme complex which catalyses the first two steps of tryptophan biosynthesis than in its free uncomplexed form, which the PRT of strain LT2 shows the same apparent Km for this substrate in both its free and complexed forms. These results confirm and extend the finding of Stuttard (1975) that strains LT2 and LT7 differ genetically form each other at a single site within region II of the trpB gene.