Conservation of the Regulatory Mechanisms Regulatng the Expression of Nitrogen Fixation Genes in R. meliloti and K. pneumoniae

Abstract

The free-living nitrogen fixing bacterium Klebsiella pneumoniae reduces N2 to NH4 + under conditions of NH4 + starvation and low O2 tension. The reduction of N2 is carried out by the enzyme complex nitrogenase which is comprised of polypeptides encoded by genes nifH, nifD and nifK. These three genes are organized into an operon transcribed in the direction nifH to nifK and are located within a cluster of at least 17 nif genes which are grouped into 7–8 transcription units. One of these operons, the nifLA operon, codes for regulatory proteins (for review, see ref. 1). The nifA product is involved in the activation of all of the other nif operons, while the nifL product is involved in the repression of these operons (2–7).