PROGRAMMED DNA REARRANGEMENT OF A CYANOBACTERIAL HUPL GENE IN HETEROCYSTS

Programed DNA rearrangements that occur during cellular differentiatio n are uncommon and have been described in only two prokaryotic organis ms. Here, we identify the developmentally regulated rearrangement of a hydrogenase gene in heterocysts of the cyanobacterium Anabaena sp. st rain PCC 7120. Heterocysts are terminally differentiated cells special ized for nitrogen fixation. Late during heterocyst differentiation, a 10.5-kb DNA element is excised from within the hupL gene by site-speci fic recombination between 16-bp direct repeats that flank the element. The predicted HupL polypeptide is homologous to the large subunit of [NiFe] uptake hydrogenases. hupL is expressed similarly to the nitroge n-fixation genes; hupL message was detected only during the late stage s of heterocyst development. An open reading frame, named xisC, identi fied near one end of the hupL DNA element is presumed to encode the el ement's site-specific recombinase. The predicted XisC polypeptide is h omologous with the Anabaena sp. strain PCC 7120 site-specific recombin ase XisA. Neither XisC nor XisA shows sequence similarity to other pro teins, suggesting that they represent a different class of site-specif ic recombinase.