Regulation of expression of nif and hut operons in Klebsiella pneumoniae by glnA linked genes of Escherichia coli

Posttranscriptional control of Klebsiella pneumoniae nif mRNA stability by the nifL product

Posttranscriptional control of nif mRNA stability was demonstrated by functional and chemical analyses, using specific probes for four nif transcripts. In the wild type, nif transcripts (except nifLA) were stable during derepression, with half-lives of approximately 30 min. They were dramatically destabilized by O2 or elevated temperature (41 degrees C) and to a lesser extent by NH4+. In contrast, the nifLA message was not particularly stable, and posttranscriptional control was not evident. In NifL- strains, both forms of analysis indicated that the nifL product was involved in nif mRNA destabilization in the presence of O2 and NH4+.

Control of Klebsiella pneumoniae nif mRNA synthesis

Four probes, each specific for a single nif transcript, were used for an analysis of the regulation of nif mRNA synthesis. Transcription of the nifLA operon was repressed by NH4+ but not by amino acids, O2, or temperatures above 37 degrees C. The nifA gene product was required for the activation of transcription of the other nif operons but not nifLA. Synthesis of the other nif transcripts was rapidly turned off by the addition of O2, NH4+, serine, or glutamine. These regulatory effects required the nifL product. However, the nifL product was not required for the cessation of synthesis of these transcripts at elevated temperatures.

Repression of nitrogen fixation in Klebsiella pneumoniae at high temperature

The effect of elevated growth temperature on nif mRNA synthesis, nif protein synthesis, and nitrogenase activity was investigated in wild-type and NifL- strains of Klebsiella pneumoniae. Nitrogenase activity is not affected at 41 degrees C; however, nitrogenase is not synthesized at that temperature. Transcription of three nif operons studied is repressed at 41 degrees C. We show that the nifL protein is not required for repression by high temperature and propose that repression of nif at 41 degrees C results from a reversible inactivation of the nifA gene product.