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

The psaI gene encoding the 4 kDa polypeptide subunit (PsaI) of the photosystem I was cloned from the Synechocystis sp. PCC 6803, a readily transformable cyanobacterium. The gene is located immediately downstream of the psaL gene in this cyanobacterial genome and encodes a polypeptide of 40 amino acid residues with a deduced molecular mass of 4,413 Da. The deduced amino acid sequence is homologous to PsaI from other cyanobacterial and higher plants. The poly-peptide has an internal hydrophobic region with a hydrophilic C-terminal region. The gene is present as a single copy in the genome. While some of its transcripts are expressed as polycistronic mRNAs which are co-transcribed with psaL, the other may be expressed as monocistronic mRNAs. Stable mutant strains in which the gene psaI was interrupted by a gene conferring resistance to kanamycin, were generated by targeted mutagenesis. One of the mutants was characterized in detail. SDS polyacrylamide gel electrophoresis revealed that the mutant strain contained other polypeptides of photosystem I in thylakoid membranes. Growth and photosynthetic characteristics of the mutant strain under photoautotrophic conditions were similar to those of the wild type, suggesting a dispensable unknown role of PsaI for photosynthesis in Synechocystis sp. PCC 6803.