Photosystem II (PSII) is a water-plastoquinone oxidoreductase and is central to the process of oxygenic photosynthesis. It is associated with a peripheral light-harvesting antenna that primarily functions in light absorption and transfer of excitation energy to the PSII reaction center (RC), where charge separation occurs. Photosynthetic productivity is in part determined by the light utilization capacity of algal cells. Wild-type algae light saturate photosynthesis at only 25% of full sunlight intensity which leads to low productivities under high light. It was our objective to increase light utilization efficiency and productivities of algal cultures by reducing/optimizing Chl b synthesis and the size of the PSII peripheral antenna. RNAi-mediated silencing of the Chl b synthesis gene, CAO, resulted in transgenics that had reduced PSII peripheral antenna sizes, up to ~2 fold higher light-saturated photosynthesis rates and 35% greater growth at high light intensities, while photoautotrophic growth at low light intensities was unimpaired. In addition, the dynamic modulation of PSII antenna size was obtained through the post-transcriptional regulation of Chl b synthesis using the light-responsive translational inhibitor protein, NAB1.
Photosynthetic productivity is also dependent on the redox properties and energetic gaps between electron donors and acceptors in PSII which in turn can be influenced by the protein microenvironment. We modified the local protein environment of ChlD1, the likely primary donor in PS II, to elucidate its function in charge separation and to study the effects of the protein environment on its redox properties. Analysis of the resulting transgenics indicated that the mutagenesis of D1-T179 to D, N or H induces a shift in the Em of P680/P680+ to more positive values, causing an acceleration of charge recombination due to a decrease in the energetic gap between the primary radical pair and the S2QA-(QB-) state. Our results show that the nature of the D1-179 residue is important in mediating excitation energy transfer to the RC and in determining the redox properties of the primary electron donor in PSII. The primary electron acceptor in PSII is PheoD1 and its redox properties are modulated by the D1-E130 amino acid residue in Chlamydomonas which serves as a hydrogen-bond donor to the PheoD1 head group. D2-Q129 is the inactive branch residue analogous to D1-E130 and is found to be too distant from the PheoD2 head group to serve as a viable hydrogen-bond donor. Our objective was to characterize the function of this highly conserved inactive branch residue by replacing it with a non-conservative leucine or a conservative histidine residue. The mutagenesis of D2-Q129 was shown to decrease the redox gap between QA and QB due to a lowering of the redox potential of QB. Further, an increased yield of S2QB- charge recombination was seen in the D2-Q129 mutants, which led to an increased susceptibility to photoinhibitory light presumably due to 3P680-mediated oxidative damage. This study provides insight into the extent of asymmetry between the two electron transfer branches in PSII where analogous active (D1-E130L) and inactive (D2-Q129L) branch mutations can impact non-analogous cofactors of the two branches.