Rearrangements of nitrogen fixation (nif) genes in the heterocystous cyanobacteria

In the vegetative cells of heterocystous cyanobacteria, such asAnabaena, two Operons harbouring the nitrogen fixaton (nif) genes contain two separate intervening DNA elements resulting in the dispersion of genes and impaired gene expression. A 11 kb element disrupts thenifD gene in thenifH, D-K operon. It contains a 11 bp sequence (GGATTACTCCG) directly repeated at its ends and harbours a gene,xisA, which encodes a site-specific recombinase. A large 55 kb element interrupts thefdxN gene in thenifB fdxN-nifS-nifU operon. It contains two 5 bp direct repeats (TATTC) at its ends and accommodates at least one gene,xisF, which encodes another site-specific recombinase. During heterocyst differentiation both the discontinuities are precisely excised by two distinct site-specific recombination events. One of them is brought about by the XisA protein between the 11 bp direct repeats. The second one is caused by the XisF protein and occurs between the 5 bp direct repeats. As a consequence the 11kb and 55 kb elements are removed from the chromosome as circles and functionalnif Operons are created. Nitrogenase proteins are then expressed from the rearranged genes in heterocysts and aerobic nitrogen fixation ensues. How these elements intruded thenif genes and how and why are they maintained in heterocystous cyanobacteria are exciting puzzles engaging considerable research effort currently. The unique developmental regulation of these gene rearrangements in heterocystous cyanobacteria is discussed.