UTP/CTP ratio, an important regulatory parameter for ATCase expression

Intracellular nucleotides of Salmonella typhimurium were separated and quantified by high performance liquid chromatography (HPLC). Wild type and specially constructed strains of S. typhimurium, in which uridine and cytidine nucleotides could be manipulated independently, were used in this study. By varying growth conditions it was possible to create different concentrations of uridine and cytidine nucleotides in the cell. The specific activity of ATCase was determined for each condition. Generally, a direct correlation was found: at high nucleotide (UTP) concentrations, maximal repression of ATCase was usually seen; at low nucleotide (UTP) concentrations ATCase was derepressed. However, it was the ratio of the concentrations of UTP-to-CTP rather than either the concentration of UTP or CTP alone that best determined the extent of ATCase expression. This applied to all conditions in the present work as well as to all conditions in work hitherto reported by others. The ratio of UTP/CTP is proposed as a key regulatory parameter for pyr enzyme expression.