TARGETED INACTIVATION OF THE GENE PSAL ENCODING A SUBUNIT OF PHOTOSYSTEM-I OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC 6803

Photosystem I is a multisubunit pigment-protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase in thylakoid membranes of cyanobacteria and higher plants. A 16-kDa protein subuni t of photosystem I complex was isolated from the cyanobacterium Synech ocystis sp. PCC 6803. The sequence of its NH2-terminal residues was de termined and a corresponding oligonucleotide probe was used to isolate the gene encoding this subunit. The gene, designated as psaL, codes f or a protein of 16,605 Da. The deduced amino acid sequence is homologo us to the subunit PsaL of barley photosystem I. There are two conserve d hydrophobic regions in the subunit PsaL that may cross or interact w ith thylakoid membranes. The gene psaL exists as a single copy in the genome and is expressed as a monocistronic RNA. Stable mutant strains in which the gene psaL was interrupted by a gene conferring resistance to chloramphenicol, were generated by targeted mutagenesis. Growth an d photosynthetic characteristics of a selected mutant strain under pho toautotrophic conditions were similar to those of the wild type, sugge sting that the function of PsaL is dispensable for photosynthesis in S ynechocystis sp. PCC 6803. Western analysis and subunit composition of purified photosystem I revealed that the mutant strain contained othe r subunits of photosystem I in thylakoid membranes and in the assemble d complex. When photosystem II activity was inhibited and glucose was supplied in the medium, mutant strains grew faster than the wild type. Under these conditions of growth, re-reduction of P700 in the mutant cells, but not in the wild type cells, showed a component with an unch aracteristically rapid half-time.