Regulation of the nitrate reductase operon: effect of mutations in chlA, B, D and E genes.

Introduction of chlA, B or E mutant alleles into strains carrying fusions between the lac structural genes and the promoter of the nitrate reductase operon led to the partial or total constitutive expression of the fusion. Presence of chlD mutated alleles in the same strains did not result in constitutive expression of the fusion and allowed full induction by nitrate only in the presence of molybdenum. It is proposed that the molybdenum cofactor, Mo-X, of the nitrate reductase is also corepressor of the operon. The chlA, B and E genes would be involved in the biosynthesis of the X-moity. Mutations in these genes would give an altered X-moity which still binds to molybdenum but leads to a less efficient repressor complex; chlD gene would code for an enzyme inserting molybdenum in the X-moity of the cofactor. Mutations in chlD give an empty cofactor leading to a complex which permanently represses the operon unless molybdenum is added.