LIGHT-REGULATED EXPRESSION OF THE PSBD GENE FAMILY IN SYNECHOCOCCUS-SP STRAIN PCC-7942 - EVIDENCE FOR THE ROLE OF DUPLICATED PSBD GENES IN CYANOBACTERIA

The genome of the cyanobacterium Synechococcus sp. strain PCC 7942 con tains two psbD genes encoding the D2 protein of the photosystem II rea ction center: psbDI, which is cotranscribed as a discistronic message with psbC (the gene encoding CP43, a chlorophyll-a binding protein), a nd psbDII, which is monocistronic. Northern blot analysis of psbD tran scripts showed that the two genes responded differently when wild-type cells were shifted from moderate to high light intensity. Whereas psb DII transcripts increased 500% relative to unshifted control cells, ps bDI-psbC transcripts remained unchanged. The beta-galactosidase activi ties expressed from translational fusions between the psbD genes and t he Escherichia coli lacZ reporter gene displayed responses similar to those seen in the RNA. D2 protein levels in thylakoid membranes from w ild-type cells increased to 250% of those of the unshifted control cel ls 12 h after a shift to high light intensities. In contrast, in a mut ant strain (AMC016) that carries an inactive psbDII gene, D2 levels de creased by 50% under identical conditions. These results suggested tha t induction of psbDII gene expression by light can serve as a suppleme ntary system for maintaining a functional photosystem II reaction cent er at high light intensity. This hypothesis was corroborated by mixed- culture experiments, in which AMC016 cells competed poorly with wild-t ype cells at high light intensity. These data suggest for the first ti me that differential expression of members of a cyanobacterial gene fa mily serves to maintain a functional PSII reaction center under divers e environmental conditions.