Programed DNA rearrangements that occur during cellular differentiation are uncommon and have been described in only two prokaryotic organisms. Here, we identify the developmentally regulated rearrangement of a hydrogenase gene in heterocysts of the cyanobacterium Anabaena sp. strain PCC 7120. Heterocysts are terminally differentiated cells specialized for nitrogen fixation. Late during heterocyst differentiation, a 10.5-kb DNA element is excised from within the hupL gene by site-specific 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
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