The carbon metabolism-controlled Synechocystis gap2 gene harbours a conserved enhancer element and a Gram-positive-like -16 promoter box retained in some chloroplast genes.

The two glyceraldehyde-3-phosphate dehydrogenase-encoding genes (gap) of Synechocystis were shown to be expressed as monocistronic transcripts. Whereas gap1 expression is slow and weak, gap2 gene induction is rapid and strong. Transcription of the gap2 gene was shown to depend on functional photosynthetic electron transport and on active carbon metabolism. The basal promoter of gap2 (P, -45 to +34, relative to the transcription start site) is controlled by three cis-acting elements designated A (-443 to -45), B (+34 to +50, in the untranslated leader region) and C (+50 to +167, in the coding region) that, together, promote a 100-fold stimulation of P activity. Element B was found to behave as a transcriptional enhancer, in that it was active regardless of its position, orientation and distance relative to P. All three cis-acting stimulatory elements exhibit a common 5'-agaTYAACg-3' nucleotide motif that appears to be conserved in cyanobacteria and may be the target for a transcriptional enhancer. We also report that gap2 transcription depends on a Gram-positive-like -16 promoter box (5'-TRTG-3') that was obviously conserved throughout the evolution of chloroplasts. This is the first report on the occurrence of a -16 promoter element in photoautotrophic organisms.