title: Expression of psbA genes is regulated at multiple levels in the cyanobacterium Synechococcus sp. PCC 7942.
abstract: In cyanobacterium Synechococcus sp. PCC 7942 the photosystem II reaction-center protein D1 is encoded by three psbA genes. The psbAI gene encodes D1:1 protein, the form used for acclimated growth, and psbAII and psbAIII genes encode the stress-induced form, D1:2 protein. Strong light and low temperature have been shown to induce the expression of psbAII/III genes and down-regulate the expression of psbAI gene. Recently, we reported the involvement of reduced thiols in the up-regulation of psbAII/III genes. In this study, we have analyzed the regulation of psbA gene expression in Synechococcus further, at both the transcriptional and post-transcriptional levels. We show that the inhibitors of the photosynthetic electron-transfer chain, which have different effects on the redox state of the plastoquinone (PQ) pool, have similar effect on the transcription of psbA genes. The inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) do not cause any changes in psbA gene expression when added under low-light conditions, but dramatically reduce the high-light induction of psbAII/III genes when added upon a high-light shift. Moreover, when the thiol reductant, dithiothreitol, is added to Synechococcus cells together with DCMU concomitant with the high-light shift, no inhibition of psbAII/III gene up-regulation takes place, indicating that the thiol redox state rather than the redox state of the PQ pool regulates psbA gene transcription. We also provide evidence for post-transcriptional regulation of psbA gene expression, particularly, inhibition of translation of psbAI transcripts at high light, and demonstrate that the D1 protein synthesis and degradation processes are coregulated in Synechococcus.
authors.affiliation: Department of Biology, University of Turku, Finland.
authors.forename: K
authors.initials: K
authors.lastname: Sippola
authors.forename: E M
authors.initials: EM
authors.lastname: Aro
mesh_terms: D000458:Cyanobacteria / Q000235:genetics*; D015964:Gene Expression Regulation, Bacterial; D008027:Light; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics*; D045332:Photosystem II Protein Complex; D012333:RNA, Messenger / Q000235:genetics; D014158:Transcription, Genetic
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D012333:RNA, Messenger