Rearrangement of nif genes during cyanobacterial heterocyst differentiation

Abstract

Anabaena is a filamentous cyanobacterium that produces specialized cells, called heterocysts, at regular intervals along each filament when deprived of fixed nitrogen under aerobic conditions. Heterocysts are anaerobic factories for nitrogen fixation. In Anabaena vegetative-cell DNA, the nifD gene, encoding the α subunit of nitrogenase, is interrupted by an 11 000 base pair DNA element. During the differentiation of heterocysts from vegetative cells, this 11 kilobase (kb) element is excised by site-specific recombination between short, directly repeated DNA sequences present at the ends of the element. The excision results in restoration of the nifD coding sequence and of the entire nifHDK transcription unit. A gene has been identified, within the 11 kb element, that is believed to encode the site-specific recombinase responsible for excision of the element during heterocyst differentiation. A second developmentally regulated gene arrangement has also been observed in Anabaena . This event occurs close to the nifS gene and involves a different set of repeated sequences, implying a different site-specific recombination system.