Repression of enzyme synthesis of the pyrimidine pathway in Salmonella typhimurium

It has been reported by other workers that a uridine and probably also a cytidine nucleotide are required for maximal repression of aspartate transcarbamylase encoded by the gene pyrB in Salmonella typhimurium. We have identified the repressing metabolites for three more biosynthetic enzymes, namely, dihydroorotate dehydrogenase (encoded by pyrD), orotidine-5'-monophosphate pyrophosphorylase (encoded by pyrE), and orotidine-5'-monophosphate decarboxylase (encoded by pyrF), as well as examining the repression profiles of aspartate transcarbamylase in more detail. Using a specially constructed strain of S. typhimurium (JL1055) which lacks the enzymes for the interconversion of cytidine and uridine compounds, thus allowing the independent manipulation of endogenous cytidine and uridine nucleotides, we found that a cytidine compound is the primary effector of repression in all cases except for aspartate transcarbamylase where little repression is observed in excess cytidine. For aspartate transcarbamylase, we found that the primary repressing metabolite is a uridine compound.