The patB gene product, required for growth of the cyanobacterium Anabaena sp. strain PCC 7120 under nitrogen-limiting conditions, contains ferredoxin and helix-turn-helix domains.

A mutant of Anabaena sp. strain PCC 7120, called PAT-2, that grows poorly under nitrogen-fixing conditions, has been isolated. The heterocysts of the mutant strain develop much more slowly than those of the wild type and are spaced more closely in an older culture of the mutant than in the wild type. The wild-type gene that complements the mutation in PAT-2, called patB, was isolated and characterized. The predicted 529-amino-acid PatB protein contains a region very similar to the Fe4S4 bacterial-type ferredoxins near its N terminus and a helix-turn-helix motif near its C terminus. This pattern of domains resembles those of transcriptional regulators in both prokaryotes and eukaryotes. The mutation in strain PAT-2 is the deletion of G at nucleotide 1342 in the patB gene, resulting in the loss of a 62-amino-acid fragment from the C terminus of the PatB protein, including the helix-turn-helix motif.