Nitrogen fixation in filamentous cyanobacteria
1991
Haselkorn, R. | Basche, M. | Böhme, H. | Borthakur, D. | Borthakur, P.B. | Buikema, W.J. | Mulligan, M.E. | Norris, D.
Filamentous cyanobacteria such as Anabaena differentiate cells specialized for nitrogen fixation, called heterocysts. A large number of mutants of Anabaena 7120, defective in heterocyst differentiation, were isolated. For some of these, the gene that complements the mutation has been identified by conjugation with a cosmid library of wild-type DNA fragments. One early-acting gene has been sequenced. This gene, hetR, is required to initiate differentiation, has no function in vegetative cells, and when present in extra copies in wild-type cells, causes the formation of multiple heterocysts. The organization of the genes for nitrogen fixation in Anabaena was also studied. The nif gene region of vegetative cell DNA of Anabaena 7120 carries two interrupting elements: an 11-kb element inserted near the carboxy terminus of the nifD gene and a 55-kb element inserted near the nifS gene. Both elements are flanked by directly repeated sequences within which site-specific recombination occurs during heterocyst differentiation. The result of these recombinations, in each case, is precise excision of the element and fusion of the heterocyst chromosome. In the case of the 11-kb element, excision leaves behind the intact nifHDK operon. The other rearrangement yields the nifB fdxN nifSU operon, where nifBSU have the same significance as their Klebsiella counterparts.
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