RESUMO
The Synechocystis PCC6803 secA gene was found to be essential for cell viability and to be transcriptionally controlled by the redox state of the cells. The basic promoter (BP, -71 to +47 relative to the transcription start site) is controlled by three cis-acting elements, which together mediate the fourfold light induction of BP activity. The positively acting element (PE1, -361 to -71) upstream of BP exerts a twofold stimulation of BP; the negative element (NE, +47 to +104) downstream of BP decreases BP strength about sixfold. The PE2 element (+104 to +175) lying in the coding sequence overcomes NE-dependent downregulation of BP. BP harbours Escherichia coli sigma70-like promoter elements -35 (5'-TTGAat-3') and -10 (5'-TAagAT-3'). The -10 motif, which has the features of an 'extended -10' box, is absolutely essential to promoter activity. The -35 hexamer is critical to the enhancement of promoter strength above BP level and to light inducibility, both features involving regulatory elements flanking BP. Most interestingly, reducing the length of the 30 bp spacing between the -35 and -10 boxes down to 17 bp was found to increase promoter activity and to confer light inducibility to BP. This demonstrates that promoter element spacing controls basal expression and light inducibility of the secA gene.