Transcriptional and posttranscriptional regulation of the psbA gene family in the cyanobacterium Synechocystis 6803

Sammanfattning: Photosynthesis is the process by which photosynthetic organisms convert solar energy into chemical energy and further into carbohydrates. In plants, algae and the prokaryotic cyanobacteria the harness and conversion of sunlight takes place in specialized membranes called thylakoids. The photosynthetic electron transport chain in the thylakoid membranes consists of several protein-pigment complexes, two of which are Photosystem II and Photosystem I. Photosystem II couples the reduction of quinone electron carriers to the oxidation of HjO and thereby produces the C>2 in the atmosphere. At the very heart of Photosystem II is a heterodimer of the D1 and D2 polypeptides. They harbor many of the key compounds involved in Photosystem II activity and are the descendants of the more primitive L and M subunits in photosynthetic reaction centers of photosynthetic bacteria. The D1 and D2 polypeptides are encoded by the psbA and psbD genes, respectively. In plants and algae these genes are located on the plastid genome and are normally present in one unique copy per genome. In cyanobacteria, on the other hand, psbA and psbD belong to small multigene families. Due to a high turnover in light the synthesis of the D1 polypeptide is stimulated by light in a process that involves transcriptional and/or translational regulation. We are studying the molecular mechanisms underlying the transcriptional regulation of psbA gene expression in the cyanobacterium Synechocystis 6803. In Synechocystis 6803 there are three psbA genes. We have demonstrated that two of the genes, psbA-2 and psbA-3 are transcribed in a light-regulated manner while the third copy, psbA-1, is not transcribed at all. The light-stimulated psbA transcription is not dependent on the photosynthetic electron transport. We also found that under normal conditions 97% of the psbA transcripts in Synechocystis 6803 originate from psbA-2 and 3% from psbA-3. After inactivation of psbA-2 the activity of psbA-3 is upregulated by a factor of 8. The stability of the psbA transcripts was found to be remarkably high in darkness with a tj^, of 7 h, as opposed to 13 min in the light. High psbA transcript stability could be obtained also in the light provided that photosynthetic electron transport was shut down. Concomitant with the high stability was the accumulation of a specific psbA transcript decay intermediate. The three psbA genes are quite homologous and the psbA-2 and psbA-3 genes encode an identical D1 polypeptide. The sequence of the psbA-2 and psbA-3 genes diverge in the 5' untranslated region. The transcription start for the psbA-2 and psbA-3 genes was mapped to, respectively, positions -49 and -87 relative to the ATG site. The psbA-2 and psbA-3 genes have identical -35 promoter sequences but differ in their -10 motif. The promoter sequences for all three psbA genes, as well as for other Synechocystis 6803 genes analyzed, conform well to E. coli consensus motifs.