Role of NtcB in activation of nitrate assimilation genes in the cyanobacterium Synechocystis sp strain PCC 6803

In Synechocystis sp. strain PCC 6803, the genes encoding the proteins invol ved in nitrate assimilation are organized into two transcription units, nrt ABCD-narB and nirA, the expression of which was repressed by ammonium and i nduced by inhibition of ammonium assimilation, suggesting involvement of Nt cA in the transcriptional regulation. Under inducing conditions, expression of the two transcription units was enhanced by nitrite, suggesting regulat ion by NtcB, the nitrite-responsive transcriptional enhancer we previously identified in Synechococcus sp. strain PCC 7942. The slr0395 gene, which en codes a protein 47% identical to Synechococcus NtcB, was identified as the Synechocystis ntcB gene, on the basis of the inability of an slr0395 mutant to rapidly accumulate the transcripts of the nitrate assimilation genes up on induction and to respond to nitrite. While Synechococcus NtcB strictly r equires nitrite for its action, Synechocystis NtcB enhanced transcription s ignificantly even in the absence of nitrite. Whereas the Synechococcus ntcB mutant expresses the nitrate assimilation genes to a significant level in an NtcA-dependent manner, the Synechocystis ntcB mutant showed only low-lev el expression of the nitrate assimilation genes, indicating that NtcA by it self cannot efficiently promote expression of these genes in Synechocystis. Activities of the nitrate assimilation enzymes in the Synechocystis ntcB m utant were consequently low, being 40 to 50% of the wild-type level, and th e cells grew on nitrate at a rate approximately threefold lower than that o f the wild-type strain. These results showed that the contribution of NtcB to the expression of nitrate assimilation capability varies considerably am ong different strains of cyanobacteria.