Convergence of two global transcriptional regulators on nitrogen inductionof the stress-acclimation gene nblA in the cyanobacterium Synechococcus spPCC 7942

Cyanobacteria respond to environmental stress conditions by degrading their phycobilisomes, the light harvesting complexes for photosynthesis. The exp ression of nblA, a key gene in this process, is controlled by the response regulator NbIR in Synechococcus sp. PCC 7942. Here we show that, under nitr ogen stress, nblA is also regulated by NtcA, the global regulator for nitro gen control. NtcA activation of nblA was found to be nitrogen-specific and did not take place under sulphur stress. Transcripts from the two major tra nscription start points (tsp) for the nblA gene were induced in response to nitrogen and sulphur starvation. The most active one (tspII) required both NbIR and NtcA to induce full nblA expression under nitrogen starvation. Nb IR and NtcA bound in vitro to a DNA fragment from the nblA promoter region, suggesting that, under nitrogen stress, both NbIR and NtcA activate the ma in regulated promoter (PnblA-2) by direct DNA-binding. The structure of Pnb lA-2 differs from that of the canonical NtcA-activated promoter and it is t herefore proposed to represent a novel type of NtcA-dependent promoter. We analysed expression patterns from ntcA and selected NtcA targets in NtcA(-) , NbIR- and wild-type strains, and discuss data suggesting further interrel ations between phycobilisome degradation and nitrogen assimilation regulato ry pathways.