CIS-ACTING SEQUENCES REQUIRED FOR NTCB-DEPENDENT, NITRITE-RESPONSIVE POSITIVE REGULATION OF THE NITRATE ASSIMILATION OPERON IN THE CYANOBACTERIUM SYNECHOCOCCUS SP. STRAIN PCC-7942

There are three binding sites for NtcA (nirI, nirII, and nirIII), the global nitrogen regulator of cyanobacteria, in the DNA region between the two divergently transcribed operons (nirA, and nirB operons) invol ved in nitrate assimilation in Synechococcus sp. strain PCC 7942. Usin g the luxAB reporter system, we showed that nirI and nirIII, which are located 23 bp upstream from the -10 promoter element of nirA and nirB , respectively, are required for induction by nitrogen depletion of th e nirA and nirB operons, respectively. The induction of nirA operon tr anscription was a prerequisite for the nitrite-responsive positive reg ulation of the transcription by NtcB, a LysR-type protein. The NtcA-bi nding site nirII, located in the middle of the nirA-nirB intergenic re gion, and a potential binding site for a LysR-type protein (TGCAN(5)TG CA; designated L1), located between nirI and nirII, were required for the nitrite-responsive, NtcB-dependent enhancement of nirA operon tran scription. Although the requirement for the L1 site was consistent wit h the involvement of the LysR family protein NtcB in transcriptional r egulation, NtcB did not bind to the nitA regulatory region in vitro in the presence of nitrite and NtcA, suggesting the involvement of some additional factor(s) in the regulation. An L1-like inverted repeat wit h the consensus sequence TGCN(7)GCA was conserved in the nirA promoter region of cyanobacteria, being centered at position -23 with respect to the NtcA-binding site corresponding to nirI,which suggested the com mon occurrence of nitrite-responsive regulation of the nitrate assimil ation operon among cyanobacteria.