Intragenic complementation by the nifJ-coded protein of Klebsiella pneumoniae

A single mutation, nifC1005 (Jin et al. Sci. Sin. 23:108-118, 1980), located between nifH and nifJ in the nif cluster of Klebsiella pneumoniae, genetically complemented mutations in each of the 17 known nif genes. This suggested that the mutation is located in a new nif gene. We showed by complementation analyses that only 3 of 12 nifJ mutations tested were complemented by nifC1005. Nitrogenase activity in cell extracts of the mutant with nifC1005 as well as NifJ- mutants was stimulated by the addition of the iron-molybdenum cofactor or nitrogenase component I. The molecular weight of the native NifJ protein is approximately 257,000--a dimer of identical subunits. Some nifC-/nifJ- or nifJ-/nifJ- merodiploids produced active but unstable nifJ proteins. Fine-structure mapping placed the nifC1005 allele within the nifJ gene bounded on both sides by well-characterized nifJ mutations. This indicates that the nifC1005 does not define a separate gene from nifJ. The data are consistent with the occurrence of intragenic complementation between two defective nifJ polypeptides. This explains the isolated examples of genetic complementation between the nifC1005 mutation and certain nifJ mutations.