Biochemical and genetic studies of a histidine regulatory mutant of Streptomyces coelicolor A3 (2)

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Abstract
The specific activities of three enzymes in a histidine regulatory mutant RF59 of Streptomyces coelicolor A3(2) resistant to the histidine analogue 1,2,4-triazolealanine (TRA) were measured and compared to the activity of the wild type strain. The first enzyme of the histidine pathway, phosphorybosyl-ATP-pyrophosphorylase (PR-ATP-pyrophosphorylase), of mutant RF59 and the wild type was sensitive to allosteric inhibition by L-histidine and hence feed-back inhibition was not affected by mutation, although the specific activity in the mutant was 2.9 fold higher than in the wild type. The other two enzymes coded by genes from the histidine operon were significantly derepressed. The enzyme D-erythroimidazoleglycerol phosphate dehydrase (IGP-dehydrase) in mutant RF59 had a 4.9 fold higher specific activity than in the wild type strain. The specific activity of the last enzyme of the pathway, histidinol-dehydrogenase (Hol-dehydrogenase), in the mutant was 4.7 fold derepressed compared to the wild type strain. The results of genetic crosses revealed the mapping of RF59 regulatory mutation between argA1 and cysD18 on S. coelicolor chromosome, suggesting that the mutant RF59 is a regulatory mutant unable to fully repress genes of the histidine pathway.

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