The polar effect on nifM of mutations in the nifU,-S,-V genes of Klebsiella pneumoniae depends on their plasmid or chromosomal location.

Polar mutations, mostly Mu and Tn insertions, were used to study the transcriptional organization of the nif gene cluster of Klebsiella pneumoniae, but there remained some ambiguity in the operon structure of the nifF, -M, -V, -S, -U genes. Complementation data presented here confirm that nifUSVM is a single operon transcribed from nifU to nifM. However, the polar effect on nifM of point, insertion and deletion mutations in nifU, -S, and -V is stronger when the mutation is on the chromosome rather than on a plasmid.