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

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.