Nucleotide sequence and expression of the two genes encoding D2 protein and the single gene encoding the CP43 protein...

The unicellular photoheterotrophic cyanobacterium Synechococcus sp. PCC 7002 was shown to encode two genes for the Photosystem II reaction center core protein D2 and one gene for the reaction center chlorophyhll-binding protein CP43. These three genes were cloned and their DNA sequences determined along with their flanking DNA sequences. Northern hybridization experiments show that both genes which encode D2, psbD1 and psbD2, are expressed at roughly equivalent levels. For each of the two psbD genes, there are 18 nucleotide differences among the 1059 nucleotides which are translated. The DNA sequences surrounding the coding sequences are nearly 70% divergent. Despite the DNA sequence differences in the genes, the proteins encoded by the two genes are predicted to be identical. The proteins encoded by psbD1 and psbD2 are ∼92% homologous to other sequenced cyanobacterial psbD genes and ∼86% homologous to sequenced chloroplast-encoded psbD genes.The single gene for CP43, psbC, overlaps the 3' end of psbD1 and is co-transcribed with it. Results from previous sequencing of psbC genes encoded by chloroplasts suggest that the 5' end of the psbC gene overlaps the 3' end of the coding sequence of psbD by ∼50 nucleotides. In Synechococcus sp. PCC 7002, the methionine codon previously proposed to be the start codon for psbC is replaced by an ACG (threonine) codon. We propose an alternative start for the psbC gene at a GTG codon 36 nucleotides downstream from the threonine codon. This GTG codon is preceded by a consensus E. coli-like ribosome binding sequence. Both the GTG start codon and its preceding ribosome binding sequence are conserved in all psbC genes sequenced from cyanobacteria and chloroplasts. This suggests that all psbC genes start at this alternative GTG codon. Based on this alternative start codon, the gene product is ∼85% identical to other cyanobacterial psbC gene products and ∼77% identical to eucaryotic chloroplast-encoded psbC gene products.