PSII-Tc Protein Plays an Important Role in Dimerization of Photosystem II

We cloned and determined the nucleotide sequence of PSII genes, psbB and psbTc, from the thermophilic cyanobacterium, Thermosynechococcus elongatus strain BP-1. PSII-Tc, encoded by psbTc, is a small membrane-spanning subunit of the PSII core complex of cyanobacteria and plants. However, its role has not been fully elucidated. We generated an insertional disruptant of psbTc and studied the role of the PSII-Tc protein in cyanobacterial PSII. The following observations were made: (i) The psbTc disruptant could grow photoautotrophically at a rate similar to that of wild-type T. elongatus under a wide range of light conditions. (ii) Thylakoids and oxygen-evolving PSII complexes were successfully isolated from the psbTc disruptant as well as the wild type. There was no significant difference in the oxygen evolution activities of cells, thylakoids or PSII complexes between the psbTc disruptant and the wild type. This is in contrast to the lower activities in the other PSII mutants of T. elongatus. (iii) Chromatographic separation of monomeric and dimeric PSII revealed that recovery of dimeric PSII was dramatically reduced in the psbTc disruptant. (iv) SDS-urea-PAGE showed a complete loss of the 4.7-kDa band in the mutant PSII. Since this band in wild-type PSII consists of PSII-M and PSII-Tc, we assume that PSII-Tc is critical for the binding of PSII-M in the PSII complex and is involved directly and indirectly in the dimerization of PSII. These results appear to be in good agreement with the recent structural model of the dimeric PSII complex.