DIFFERENTIAL EXPRESSION OF THE PSBA GENES IN THE CYANOBACTERIUM SYNECHOCYSTIS-6803

The 5' region and transcription initiation sites of the psbA-2 and psb A-3 genes of Synechocystis 6803 were determined. The otherwise highly homologous genes were shown to diverge significantly in the 5' noncodi ng regions. The transcription start site for thepsbA-2 gene was mapped to position -49 upstream of the coding region and for the psbA-3 gene to position -88, i.e. 38 bp upstream of the psbA-2 transcription star t point. Both genes exhibit promoter elements, which conform in sequen ce and position to Escherichia coli consensus motifs. The two genes sh are identical -35 sequences but differ in their -10 sequences. Primer extension analysis demonstrated that the psbA-2 and psbA-3 genes are d ifferentially expressed, with > 90 % of the total psbA transcripts bei ng produced by the psbA-2 gene and the rest by the psbA-3 gene. Inacti vation of the psbA-2 gene resulted in an eightfold up-regulation of th epsbA-3 gene. The strikingly higher stability of the psbA transcripts in darkness compared to light, and the accumulation of a specific deca y intermediate under dark conditions was reported previously. We show here that this dark-stability applies to both the psbA-2 and psbA-3 tr anscripts. The psbA-3 transcript did not appear to produce the process ed intermediate, arguing for the involvement of the 5' non-coding regi on as a determinant in psbA transcript degradation.