Targeted inactivation of the gene psaK encoding a subunit of photosystem I from the cyanobacterium Synechocystis sp. PCC 6803.

Mutant strains of the unicellular cyanobacterium Synechocystis sp. PCC 6803, in which the psaK gene was insertionally inactivated by targeted mutagenesis, were constructed. The gene is one of the two potential PsaK-coding genes which have been found as a result of the genome project with this cyanobacterium. One of the mutants was characterized in detail. A monocistronic, 480-nucleotide mRNA of psaK was absent in total RNA from the mutant cells. Inactivation of psaK had little effect on the accumulation of polypeptides in the isolated PSI complexes except for a polypeptide with an apparent molecular mass of 4.6 kDa which was absent in the mutant. The amino-terminal amino acid sequence of the 4.6-kDa polypeptide confirmed that it was the translation product of psaK and further revealed a presequence of PsaK. Characteristics of photoautotrophic growth at different temperatures, the amount of chlorophyll per cell, photosynthetic electron transport rates with rates with various electron acceptors, the kinetics of charge recombination between P700+ and reduced FA/FB, and the molar ratio of chlorophyll to P700, of the mutant were not significantly different from those of the wild type. Furthermore, the trimer to monomer ratio of the PSI complexes isolated from the mutant was similar to that isolated from the wild type.