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Investigating the Role(s) of LHCSRs in Chlamydomonas reinhardtii


Photosynthetic organisms must balance light absorption and light utilization. Excess absorbed light leads to the production of reactive oxygen species that can cause photo-oxidative damage to the cell and even cell death. To counteract the excess absorbed light, plants and algae turn on the rapid component of non-photochemical quenching (NPQ) known as qE to dissipate the extra energy as heat. From studies of higher plants, it is known that three factors are needed for the activation of qE: 1) de-epoxidized xanthophylls synthesized by a xanthophyll cycle, 2) a high proton gradient across the thylakoid membrane, and 3) the PsbS protein, a member of the light-harvesting complex (LHC) protein superfamily. Although the first two components are also involved in qE in the green alga Chlamydomonas reinhardtii, PsbS does not appear to play a role in quenching. The role of a different protein, called light-harvesting complex stress-related (LHCSR), was demonstrated by molecular genetic analysis of the npq4 mutant of Chlamydomonas. This mutant is deficient in qE and lacks two closely linked LHCSR genes (LHCSR3.1 and LHCSR3.2) that encode identical LHCSR3 proteins. A third gene, LHCSR1, encodes a second LHCSR isoform that is also involved in qE. The LHCSR1 protein is overexpressed in a suppressor of npq4 that partially restores the qE capacity of the mutant. A loss-of-function lhcsr1 mutation was isolated using reverse genetics, and an lhcsr1 npq4 double mutant exhibited no qE. Thus, all qE in Chlamydomonas is dependent on LHCSR proteins. Based on biochemical and biophysical analysis of a reconstituted LHCSR3 pigment-protein complex and site-directed mutagenesis of LHCSR3, LHCSRs have a dual function in Chlamydomonas, acting as sensors of lumen pH and as the sites of quenching, unlike in higher plants where the role of high light sensing is attributed to PsbS and the sites of quenching are in the minor and/or major antenna complexes. These findings have interesting implications for the evolution of qE in the green lineage.

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