Synechocystis sp PCC 6803 and Phycobilisome Function Strains Lacking Photosystem I

To design an in vivo system allowing detailed analysis of photosystem II (PSII) complexes without significant interference from other pigment complexes, part of the psaAB operon coding for the core proteins of photosystem I (PSI) and part of the apcE gene coding for the anchor protein linking the phycobilisome to the thylakoid membrane were deleted from the genome of the cyanobacterium Synechocystis sp strain PCC 6803. Upon transformation and segregation at low light intensity (5 pE m+ sec-l), a PSI deletion strain was obtained that is light tolerant and grows reasonably well under photoheterotrophic conditions at 5 pE m-2 sec-l (doubling time -28 hr). Subsequent inactivation of apcE by an erythromycin resistance marker led to reduction of the phycobilin-to-chlorophyll ratio and to a further decrease in light sensitivity. The resulting PSI-IesslapcEstrain grew photoheterotrophically at normal light intensity (50 pE m-2 se&) with a doubling time of 18 hr. Deletion of apcE in the wild type resulted in slow photoautotrophic growth. The remaining phycobilins in apcEstrains were inactive in transferring light energy to PSII. Cells of both the PSI-less and PSI-IesslapcEstrains had an approximately sixfold enrichment of PSll on a chlorophyll basis and were as active in oxygen evolution (on a per PSll basis) as the wild type at saturating light intensity. Both PSI-less strains described here are highly appropriate both for detailed PSll studies and as background strains to analyze siteand region-directed PSll mutants in vivo.