Abstract
Two structurally unrelated protochlorophyllide reductases, light-dependent reductase (LPOR) and dark-operative reductase (DPOR), coexist in chlorophyll biosynthesis in cyanobacteria. DPOR is a nitrogenase-like enzyme that is rapidly inactivated by exposure to oxygen. To understand how the oxygen-sensitive enzyme, DPOR operates in oxygenic photosynthetic cells, we isolated a mutant lacking LPOR in Synechocystis sp. PCC 6803. While the mutant was sensitive to high light, it grew under low light conditions, suggesting the presence of some protection system for DPOR against oxygen. Some candidate genes involved potentially in this system were artificially overexpressed at a neutral site (slr2030-slr2031) in the LPOR-lacking mutant. Unexpectedly, the light-sensitivity was relieved in a negative control transformant with an empty vector as well as in the other transformants. Genome analysis revealed that the parental strain retains the intact slr2031 coding sequence while "Kazusa" strain, whose genome has been fully sequenced, has a 154-bp deletion in slr2031. Thus, a deletion of slr2031 caused by the plasmid insertion presumably resulted in the tolerance to high light intensity.
