The NtcA-dependent P1 promoter is utilized for glnA expression in N2-fixing heterocysts of Anabaena sp. strain PCC 7120.

Expression of the glnA gene encoding glutamine synthetase, a key enzyme in nitrogen metabolism, is subject to a variety of regulatory mechanisms in different organisms. In the filamentous, N(2)-fixing cyanobacterium Anabaena sp. strain PCC 7120, glnA is expressed from multiple promoters that generate several transcripts whose abundance is influenced by NtcA, the transcription factor exerting global nitrogen control in cyanobacteria. Whereas RNA(I) originates from a canonical NtcA-dependent promoter (P(1)) and RNA(II) originates from a sigma(70)-type promoter (P(2)), RNA(IV) is influenced by NtcA but the corresponding promoter (P(3)) does not have the structure of NtcA-activated promoters. Using RNA isolated from Anabaena filaments grown under different nitrogen regimens, we observed, in addition to these transcripts, RNA(V), which has previously been detected only in in vitro transcription assays and should originate from P(4). However, in heterocysts, which are differentiated cells specialized in N(2) fixation, RNA(I) was the almost exclusive glnA transcript. Analysis of P(glnA)::lacZ fusions containing different fragments of the glnA upstream region confirmed that fragments carrying P(1), P(2), or P(3) and P(4) have the ability to promote transcription. Mutation of the NtcA-binding site in P(1) eliminated P(1)-directed transcription and allowed increased use of P(2). The NtcA-binding site in the P(1) promoter and binding of NtcA to this site appear to be key factors in determining glnA gene expression in vegetative cells and heterocysts.