Hie presence of five nifH-iike sequences in Oostridium 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 nlfH-like sequences in addition to the nlfHl 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 (nlfHl) and nlfH-like (nlfH2. nlfH3, nlfH4, nlfH5 and nifH6) sequences are not identical and vary from each other to different extents with sequence identity ranging between 68 to 99.9Z within the nifH coding regions. Under normal ^-fixing growth conditions (molybdenum-containing medium), transcripts of nlfHl and most of the nlfH-like sequences accumulate. The above results suggest the functioning of more than one "nifH " gene under ^-fixing growth conditions for C. pasteurianum. A common sequence was found around the-100 regions of all nlf or nlf-llke transcription units. Sequences identical to or very similar to the consensus Escherichia coll promoter were found in the-35 and-10 regions