TRANSCRIPTION OF GLUTAMINE-SYNTHETASE GENES (GLNA AND GLNN) FROM THE CYANOBACTERIUM SYNECHOCYSTIS SP STRAIN PCC-6803 IS DIFFERENTLY REGULATED IN RESPONSE TO NITROGEN AVAILABILITY

In the cyanobacterium Synechocystis sp, strain PCC 6803 we have previo usly reported the presence of two different proteins with glutamine sy nthetase activity: GSI, encoded by the glnA gene, and GSIII, encoded b y the glnN gene, In this work we show that expression of both the glnA and glnN genes is subjected to transcriptional regulation in response to changes in nitrogen availability, Northern blot experiments and tr anscriptional fusions demonstrated that the glnA gene is highly transc ribed in nitrate- or ammonium-grown cells and exhibits two- to fourfol d-higher expression in nitrogen-starved cells, In contrast, the glnN g ene is highly expressed only under nitrogen deficiency, Half-lives of both mRNAs, calculated after addition of rifampin or ammonium to nitro gen-starved cells, were not significantly different (2.5 or 3.4 min, r espectively, for glnA mRNA; 1.9 or 1.4 min, respectively, for glnN mRN A), suggesting that changes in transcript stability are not involved i n the regulation of the expression of both genes, Deletions of the gln A and glnN upstream regions were used to delimit the promoter and the regulatory sequences of both genes, Primer extension analysis show ed that structure of the glnA gene promoter resembles those of the NtcA-r egulated promoters, In addition, mobility shift assays demonstrated th at purified, Escherichia coli-expressed Synechocystis NtcA protein bin ds to the promoter of the glnA gene, Primer extension also revealed th e existence of a sequence related to the NtcA binding site upstream fr om the glnN promoter, However, E. coli-expressed NtcA failed to bind t o this site, These findings suggest that an additional modification of NtcA or an additional factor is required for the regulation of glnN g ene expression.