TARGETED INTERRUPTION OF THE PSAA AND PSAB GENES ENCODING THE REACTION-CENTER PROTEINS OF PHOTOSYSTEM-I IN THE FILAMENTOUS CYANOBACTERIUM ANABAENA-VARIABILIS ATCC-29413

The two reaction-centre proteins of the photosystem I (PSI) complex ar e encoded by two adjacent genes named psaA and psaB. We have performed targeted mutagenesis to insertionally inactivate each of these genes in the filamentous cyanobacterium Anabaena variabilis ATCC 29413. The resulting mutant strains, termed psaA::NM(R) and psaB::NM(R), were blu e because of a high ratio of phycobilin to chlorophyll and were unable to grow in light. These mutant cells also lacked chemically reducible P700 (the reaction-centre chlorophylls of PSI) and as a consequence d id not exhibit any PSI-mediated photochemical activity. However, their photosystem II (PSII) complexes were fully active. The loss of the Ps aA and PsaB proteins and their associated chlorophyll molecules result ed in a five- to sevenfold decrease in the chlorophyll/PSII ratio in t he mutant cells relative to the wild-type cells. Interestingly, the ps aB::NM(R) and not the psaA::NM(R) mutant strain retained a small fluor escence peak (77K) at 721 nm originating from chlorophyll molecule(s) presumably bound to a small amount of the PsaA protein present in the psaB mutant. These results demonstrate that this organism is suitable for the manipulation of PSI reaction-centre proteins.