CHLOROPHYLL-A SYNTHESIS UPON INTERRUPTION AND DELETION OF POR CODING FOR THE LIGHT-DEPENDENT NADPH-PROTOCHLOROPHYLLIDE OXIDOREDUCTASE IN A PHOTOSYSTEM-I-LESS CHLL(-) STRAIN OF SYNECHOCYSTIS SP. PCC-6803/

The gene coding for the light-dependent NADFH.protochlorophyllide oxid oreductase (POR) was interrupted or deleted in a Synechocystis sp. PCC 6803 strain lacking photosystem I (PS I) as well as ChlL, which takes part in light-independent catalysis of protochlorophyllide reduction. Interruption of por by a kanamycin-resistance cartridge between the c odons for M263 and V264 (about 83% into the coding region) did not abo lish POR activity, but resulted in a decrease in the protochlorophylli de-(PChlide)-binding capacity of POR. Deletion of por in the PS I-less /chlL(-) strain generated a mutant [PS I-less/chlL(-)/por (del)] which accumulated both monovinyl-PChlide and divinyl-PChlide and excreted P Chlides into the medium. This mutant also synthesized small amounts of protochlorophyllide dihydrogeranylgeraniol ester (protochlorophyll) w hen it was grown under light-activated heterotrophic growth conditions . However, the mutant was still able to synthesize small amounts of no rmal chlorophyll a under weak continuous illumination, even though the quantum yield of chlorophyll a formation was reduced. Either protochl orophyll or PChlide reduction by an unspecific reductase or by a ChlB/ ChlN complex could account for chlorophyll a synthesis in the PS I-les s/chlL(-)/por (del) strain. Functional photosystem II (PS II) was asse mbled in this mutant, but the PS II/chlorophyll ratio was fourfold low er than in the PS I-less strain with normal chlorophyll synthesis. The PS I-less/chlL(-)/por (del) mutant had a 77-K fluorescence emission m aximum at 685 nm but no peak or shoulder at 695 nm when the cells were excited at 435 nm. Much of the chlorophyll in the PS I-less/chlL(-)/p or (del) mutant therefore seems to be associated with components other than PS II.