The physiological significance of photosystem II heterogeneity in chloroplasts

Photosystem II (PS II) heterogeneity in chloroplasts is manifested in two distinct ways. The first involves the existence of PS II centers with different chlorophyll antenna sizes and is termed "PS II antenna heterogeneity". The second involves electron flow between the primary quinone electron acceptor, Q{dollar}sb{lcub}rm A{rcub}{dollar}, and the secondary quinone electron acceptor, Q{dollar}sb{lcub}rm B{rcub}{dollar} and is termed "PS II reducing side heterogeneity". The properties and physiological significance of the PS II reducing side heterogeneity and its relationship to PS II antenna heterogeneity were investigated.; Photosystem II centers with efficient electron-transport from Q{dollar}sb{lcub}rm A{rcub}{dollar} to Q{dollar}sb{lcub}rm B{rcub}{dollar} (Q{dollar}sb{lcub}rm B{rcub}{dollar}-reducing) for about 75% of the total PS II in many photosynthetic organisms. The remaining 25% of PS II centers, though photochemically competent, are unable to transfer electrons from Q{dollar}sb{lcub}rm A{rcub}{dollar} to Q{dollar}sb{lcub}rm B{rcub}{dollar} (Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing). In Dunaliella salina the pool size of the Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing centers changed transiently when the growth light regime of the cells was perturbed. Dark incubation of the cells resulted in an increase in the Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing pool size. Subsequent illumination of the cells restored the pool size to the steady-state concentration of 25%. Transfer of low-light grown cells to medium-light conditions induced a rapid decrease in the Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing pool size and a concomitant increase in the Q{dollar}sb{lcub}rm B{rcub}{dollar}-reducing pool size. The results suggested that the Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing population is dynamic and that steady-state pools of Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing, Q{dollar}sb{lcub}rm B{rcub}{dollar}-reducing and photochemically silent PS II centers exist in the thylakoid membrane. The results also implied that perturbation of the growth light conditions causes transient changes in these steady-state concentration. I propose that photosynthetic cells regulate the interconversion of the various forms of PS II in order to adapt to changes in the light environment. Work with Chlamydomonas reinhardtii grown in the dark demonstrated that newly synthesized PS II centers first appear as Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing centers and that light is required to convert these centers to the Q{dollar}sb{lcub}rm B{rcub}{dollar}-reducing form. I propose that Q{dollar}sb{lcub}rm B{rcub}{dollar}-nonreducing centers represent the first photochemically competent stage of PS II both during repair and during de novo synthesis of PS II. A model for the repair of PS II, and a model for the activation of newly synthesized PS II centers to the Q{dollar}sb{lcub}rm B{rcub}{dollar}-reducing form is presented.