Evolution and variation of the nifD and hupL elements in the heterocystous cyanobacteria

Abstract

In heterocystous cyanobacteria, heterocyst differentiation is accompanied by developmentally regulated DNA rearrangements that occur within thenifDandhupLgenes, referred to as thenifDandhupLelements. These elements are segments of DNA that are embedded within the coding region of each gene and range from 4 to 24 kb in length. ThenifDandhupLelements are independently excised from the genome during the later stages of differentiation by the site-specific recombinases, XisA and XisC, respectively, which are encoded within the elements themselves. Here we examine the variation and evolution of thenifDandhupLelements by comparing full-lengthnifDandhupLelement sequences and by phylogenetic analysis ofxisAandxisCgene sequences. There is considerable variation in the size and composition of thenifDandhupLelements, however, conserved regions are also present within representatives of each element. The data suggest that thenifDandhupLelements have undergone a complex pattern of insertions, deletions, translocations and sequence divergence over the course of evolution, but that conserved regions remain.