Organization and transcription of genes important in Anabaena heterocyst differentiation.

The structural genes for nitrogenase and nitrogenase reductase have been cloned from Anabaena and physically mapped. The map differs from that of Klebsiella in several ways, including the insertion of 11 kbp between nifK and nifD in Anabaena. One nif RNA transcript has been studied in detail and shown to originate from a site in the Anabaena chromosome which lacks good correspondence with a typical prokaryotic strong promoter, suggesting the possibility of a need for positive activation. The nifH message is unstable or repressed or both under aerobic conditions. This feature is sufficient to account for the need for heterocyst differentiation in order for Anabaena to fix nitrogen aerobically. Structural genes for glutamine synthetase and the large subunit of RuBP carboxylase were also cloned, mapped and used to study transcription. In each case, the level of messenger RNA following nitrogenase induction is consistent with regulation of these genes at the level of transcription.