The presence of five nifH-like sequences in Clostridium pasteurianum: sequence divergence and transcription properties

Abstract

The nifH gene encodes the iron protein (component II) of the nitrogenase complex. We have previously shown the presence in Clostridium pasteurianum of two nifH-like sequences in addition to the nifH1 gene which codes for a protein identical to the isolated iron protein. In the present study, we report that there are at least five nifH-like sequences in C. pasteurianum. DNA sequencing data indicate that the six nifH (nifH1) and nifH-like (nifH2, nifH3, nifH4, nifH5 and nifH6) sequences are not identical and vary from each other to different extents with sequence identity ranging between 68 to 99.9% within the nifH coding regions. Under normal N2-fixing growth conditions (molybdenum-containing medium), transcripts of nifH1 and most of the nifH-like sequences accumulate. The above results suggest the functioning of more than one "nifH" gene under N2-fixing growth conditions for C. pasteurianum. A common sequence was found around the -100 regions of all nif or nif-like transcription units. Sequences identical to or very similar to the consensus Escherichia coli promoter were found in the -35 and -10 regions.