Rearrangement of nif Operons in Cyanobacterial Heterocysts

Abstract

Cyanobacteria are a diverse group of photosynthetic prokaryotic organisms found in a wide range of environments. Some filamentous cyanobacteria exhibit a striking example of cellular differentiation to produce heterocysts, highly specialized cells that fix atmospheric nitrogen. Nitrogen-fixing cyanobacteria make significant contributions to the available nitrogen in many natural environments and supply fixed nitrogen in some agricultural settings. The differentiation of a photosynthetic vegetative cell into a nitrogen-fixing heterocyst is a complex process that is regulated by the availability of combined nitrogen and intercellular communication, and results in a semi-regular pattern of heterocysts along the vegetative cell filament. Heterocyst differentiation involves global changes in gene expression: genes for photosynthesis and carbon fixation are turned off and genes for nitrogen fixation are turned on. Two heterocyst-specific genome rearrangements are required for the correct expression of nitrogen-fixation operons. These rearrangements involve the excision of 11-kb and 55-kb DNA elements from within the nifD and fdxN genes, respectively. The xisA gene, present on the 11-kb element, encodes a site-specific recombinase that is required for the nifD rearrangement. We have identified DNA sequences upstream of xisA that may be involved in regulating xisA expression. A protein, BifA (previously called VF1), isolated from Anabaena vegetative cells binds to the xisA upstream region and to the upstream sequences of several other genes. The bifA gene was cloned using a novel in vivo selection strategy. The bifA gene was sequenced and its predicted amino acid sequence shows significant similarity to a family of transcriptional regulators represented by the E. coli Crp protein. The regulation of BifA activity and its putative function in transcriptional control are currently being studied. The xisF gene, present on the fdxN element, encodes the recombinase required for the fdxN rearrangement. XisF shows significant similarity to the Bacillus subtilis developmentally regulated site-specific recombinase encoded by the spoIVCA (cisA) gene. A reverse genetics approach involving the targeted inactivation of xisF was used to study its role in the DNA rearrangements and expression of the nif genes.