The role of the low-molecular-weight proteins of the CP43 pre-assembly complex of Photosystem II

The biogenesis of Photosystem II (PS II) involves the stepwise assembly of the reaction centre subcomplex together with the subsequent addition of the two chlorophyll-binding core antenna proteins, CP43 and CP47. In this study, the function of four low-molecular-weight proteins (Psb27, Psb30, PsbK and PsbZ) belonging to the CP43 pre-assembly complex was investigated in the cyanobacterium Synechocystis sp. PCC 6803. The PsbK, PsbZ and Psb30 polypeptides are retained in the holoenzyme, but while Psb27 appears to participate in both the biogenesis and repair pathways, it is not a constituent of the mature complex. The role of conserved amino acid residues of the Psb27 protein potentially involved in protein-protein interactions with other PS II subunits was studied by introducing amino acid substitutions in Synechocystis sp. PCC 6803. These amino acid substitutions did not impede the biogenesis and repair pathways of photosystem and PS II electron transport kinetics in these mutants resembled those observed for wild type. However, chlorophyll fluorescence induction measurements indicated mutations in Psb27 altered the state transitions regulating energy transfer to the Photosystem I (PS I) and PS II and the D14A, D58E, D58K, and K63D strains exhibited changes in PBS coupling and energy transfer to PS II. Additionally, Psb27 mutations were found to alter the PS I level. Whole genome sequencing of a particular Psb27 mutant line (R54E*) revealed a C to A substitution in the psbA2 gene corresponding to a His252 to Gln substitution in the D1 reaction centre protein. Confirmation that D1:His252 was required for normal electron transfer between the primary (QA) and secondary (QB) electron acceptors was obtained by introducing point mutations into a psbA-deletion mutant of Synechocystis sp. PCC 6803. Additionally, the D1-Ser264 was also targeted, since D1-Ser264 has also been suggested to be involved in the configuration of the QB site and is in close proximity to His252. The D1-His252 mutants as well as D1-Ser264 mutants, displayed low PS II oxygen-evolving activity and impaired electron transfer between QA and QB supporting calculations suggesting His252 and Ser264 participate in the protonation of the QB- semiquinone during the turnover of the two-electron gate. Additionally, the PsbK, PsbZ and Psb30 subunits were not absolutely required for PS II activity; however, it was found that these subunits contributed to the ability of PS II to withstand photodamage following exposure to high light. Also, the Psb27 protein appears necessary for efficient repair or photoactivation of PS II and its absence along with LMW proteins (Psb30, PsbK and PsbZ) results in an increased susceptibility to photoinhibition