Photosystem II Heterogeneity in Chloroplasts

Abstract

Two aspects of photosystem II (PSII) heterogeneity are discussed. (A) The PSII antenna heterogeneity refers to the existence of PSII centers with distinct photosynthetic unit size. Measurements of PSII antenna size in developing chloroplasts, in Chl b-less and Chl b-deficient mutants, and in mature higher plant chloroplasts indicated the existence of three distinct populations of PSII centers: PSIIα with a total of 210 (or more) chlorophyll (a+b) molecules; PSIIβ with a total of 120 Chl (a+b) molecules. In the absence of Chl b, a PSII complex containing only 50 Chl a molecules (PSII-50) was identified. A developmental relationship among PSII-50, PSIIβ and PSIIα is proposed. According to this hypothesis, the formation of the complete PSII unit involves the assembly of three modular complexes (PSII-50, LHC II-inner, and LHC II-peripheral). Addition of LHC II-inner to PSII-50 yields PSIIβ. Addition of LHC II-peripheral to PSIIβ yields PSIIα. The relative proportion of PSII centers with the three antenna configurations depends on the developmental stage of the chloroplast and on the availability of Chl b. Mature wild type chloroplasts contain PSIIα (75–80% of the total PSII) and PSIIβ (20–25% of the total PSII). (B) The PSII reducing side heterogeneity refers to the existence of PSĪI centers with impaired Q to QB electron transfer interaction (QB-nonreducing centers). The fraction of PSII-QB-nonreducing centers is small (20–25% of the total PSII) as tested in several chloroplast preparations. This steady state concentration of QB-nonreducing centers appears independent of the developmental stage of the chloroplast and also independent of the PSII photosynthetic unit size. In mature spinach chloroplasts, PSIIβ and PSII-QB-nonreducing centers constitute one and the same pool of PSII centers. It is proposed that PSII-QB-nonreducing centers represent newly synthesized and/or repaired PSII centers which have not yet established a functional interaction between QȦ and QB.