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Abstract The third and fourth enzymes of the pathway of histidine degradation in Salmonella typhimurium are induced by histidine and repressed by glucose. An improved assay for the third enzyme, 4-imidazolone-5-propionate amidohydrolase, was developed and some properties of this enzyme were studied. Mutants with decreased stability of this enzyme have been isolated. Their lesions are unlinked by transduction to the cluster containing the genes for the histidine-degrading enzymes. Mutants lacking 4-imidazolone-5-propionate amidohydrolase or the fourth enzyme, N-formimino-l-glutamate formiminohydrolase, have also been isolated. Their lesions are located in two closely linked genes near the cluster of genes controlling the synthesis of the first two enzymes of the pathway (histidase and urocanase). Mutations leading to increased levels of the third and fourth enzymes are located to one side of this pair of genes. The following observations suggest that these two clusters form separate units of expression. (a) While the two enzyme activities within each pair are coordinately controlled upon induction or catabolite repression, there is no apparent coordination between the pairs. (b) Mutations affecting the level of enzymes from 1 pair of genes and located to one side of them have no effect on the level of enzymes from the other pair. (c) A gene controlling the inducibility of all four enzymes is located between the two clusters. It is suggested that the units of expression correspond to two operons controlled by a single regulatory gene located between them.
Smith et al. (Sat,) studied this question.