A molecular genetic study of nif expression in Klebsiella pneumoniae at the level of transcription, translation and nitrogenase activity

Abstract

A comprehensive study of nif expression in Klebsiella pneumoniae at the level of transcription, translation and nitrogenase activity during derepression and repression by NH ₄ ⁺ and O₂ revealed that (1) transcription and translation rates remained coupled under all conditions; (2) these rates reached a peak during derepression and then decreased to a low level; (3) the transcription profile of nifLA had two peaks; the first was at 1 h before and the second coincided with that of the other operons; and (4) the peaks of nif transcription coincided with a trough in the profile of stringent regulation of RNA synthesis. Our results provide strong evidence that nif-specific repression by NH ₄ ⁺ and O₂ occurs exclusively by transcription inhibition and that repression by O₂ is independent of transcriptional regulation of the nifLA operon. We have also found evidence which together with the results of previous work shows that O₂ repression of nifA mediated transcription involves the nifL gene product.