Functional analysis of the two homologous psbA gene copies in Synechocystis PCC 6714 and PCC 6803.

The cyanobacteria Synechocystis 6803 and 6714 contain three genes (psbA) coding for the D1 protein. This protein is an essential subunit of photosystem II (PSII) and is the target for herbicides. We have used herbicide-resistant mutants to study the role of the two homologous copies of the psbA genes in both strains (the third copy is not expressed). Several herbicide resistance mutations map within the psbAI gene in Synechocystis 6714 (G. Ajlani et al., Plant Mol. Biol. 13 (1989): 469-479). We have looked for mutations in copy II. Results show that in Synechocystis 6714, only psbAI contains herbicide resistance mutations. Relative expression of psbAI and psbAII has been measured by analysing the proportions of resistant and sensitive D1 in the thylakoid membranes of the mutants. In normal growth conditions, 95% resistant D1 and 5% sensitive D1 were found. In high light conditions, expression of psbAII was enhanced, producing 15% sensitive D1. This enhancement is specifically due to high light and not to the decrease of D1 concentration caused by photoinhibition. Copy I of Synechocystis 6714 corresponds to copy 2 of Synechocystis 6803 since it was always psbA2 which was recombined in Synechocystis 6803 transformants. PSII of the transformant strains was found to be 95% resistant to herbicides as in resistant mutants of Synechocystis 6714.