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Summary

Heterocyst differentiation in the cyanobacterium Anabaena sp. strain PCC 7120 depends on both the global nitrogen regulator NtcA and the cell differentiation regulatory protein HetR, and induction of hetR upon nitrogen step-down depends on NtcA. The use of two out of the four transcription start points (tsps) described for the hetR gene (those located at positions –728 and –271) was found to be dependent on NtcA, and the use of the tsp located at position –271 was also dependent on HetR. Thus, autoregulation of hetR could take place via the activation of transcription from this tsp. Expression of ntcA in nitrogen-fixing cultures was higher than in cells growing in the presence of ammonium or nitrate, and high expression of ntcA under nitrogen deficiency resulted from an increased use of tsps located at positions –180 and –49. The induction of the use of these tsps did not take place in ntcA or hetR mutant strains. These results indicate a mutual dependency in the induction of the regulatory genes hetR and ntcA that takes place in response to nitrogen step-down in Anabaena cells. Expression of the hetC gene, which is also involved in the early steps of heterocyst differentiation, from its NtcA-dependent tsp was, however, not dependent on HetR.