Nitrogen fixation (nif) genes of the cyanobacterium Anabaena species strain PCC 7120. The nifB-fdxN-nifS-nifU operon.

A second nitrogen fixation (nif) operon in the cyanobacterium (blue-green alga) Anabaena (Nostoc) sp. strain PCC 7120 has been identified and sequenced. It is located just upstream of the nifHDK operon and consists of four genes in the order nifB, fdxN, nifS, and nifU. The three nif genes were identified on the basis of their similarity with the corresponding genes from other diazotrophs. The fourth gene, fdxN, codes for a bacterial type ferredoxin (Mulligan, M. E., Buikema, W. J., and Haselkorn, R. (1988) J. Bacteriol. 167, 4406-4410). The four genes are probably transcribed as a single operon, but are expressed at a lower level than the nifHDK operon, and only after a developmentally induced DNA rearrangement occurs that excises a 55-kilobase pair element from within the fdxN gene (Golden, J. W., Mulligan, M. E., and Haselkorn, R. (1987) Nature 327, 526-529; Golden, J. W., Carrasco, C. D., Mulligan, M. E., Schneider, G. J., and Haselkorn, R. (1988) J. Bacteriol. 170, 5034-5041). The promoter for the nifB operon was located by primer extension. Comparison of the nifB 5'-flanking sequence with the nifH 5'-flanking sequence did not reveal any consensus base pairs that would define a nif promoter for Anabaena. The operon contains two instances of 7-base pair directly repeated sequences: seven copies of the repeated sequence are found between the nifB and fdxN genes and six copies are found between the nifS and nifU genes. The function of these repeats is unknown.