NtcA-regulated heterocyst differentiation genes hetC and devB from Anabaena sp. strain PCC 7120 exhibit a similar tandem promoter arrangement.

Transcription of the hetC gene, whose product is required for heterocyst differentiation, takes place from a long promoter region that includes the previously described HetR-independent, NtcA-activated promoter producing transcripts with a 5' end corresponding to position -571 with respect to the translational start site of hetC. Northern blot analysis indicated that the accumulation of hetC transcripts depends on HetR, and a second transcriptional start site located at position -293 that leads to NtcA-dependent, HetR-dependent inducible transcription of hetC was identified. Upon nitrogen stepdown, expression of a P(hetC)::gfp fusion was transiently induced in specific cells that were differentiating into heterocysts, both when the whole promoter region (containing transcription start points -571 and -293) or a short version (containing only the transcription start point -293) was used. Expression of hetC from the -293 position was delayed in a strain bearing a deleted promoter region lacking sequences upstream from position -570. Such a strain was still able to differentiate functional heterocysts and to grow diazotrophically, although diazotrophic growth was impaired under certain conditions. Similarly, a second, NtcA-dependent, HetR-dependent transcriptional start site was identified at position -454 in the promoter region upstream from the devBCA operon encoding an ABC transport system involved in heterocyst maturation, in which an NtcA-dependent promoter producing transcripts starting at position -704 had been previously noted. Thus, the hetC and devBCA promoter regions exhibit similar tandem promoter arrangements.