Photoprotection and light harvesting in Chlamydomonas reinhardtii

In natural environments, pigments in light-harvesting complexes often absorb more light energy than can be utilized for carbon fixation. Under these conditions of excess excitation energy, reactive oxygen species are generated, and may damage cellular components. A major line of defense against oxidative damage during exposure to excess light is thermal dissipation of excess excitation energy in the light-harvesting complex of photosystem II. Although roles for both carotenoids and specific polypeptides in thermal dissipation have been reported, our understanding of the mechanism of thermal dissipation and of its importance for surviving high-light exposure is still limited. This thesis describes the physiological and molecular characterization of the npq5 mutant of Chlamydomonas reinhardtii, which is defective in thermal dissipation. The physiological and molecular characterizations of the npq5 mutant strain suggest that most thermal dissipation in Chlamydomonas depends on LHCIIb. The npq5 mutant strain is wild-type for photoautotrophic growth, state transition, and high-light induced violaxanthin de-epoxidation, but accumulates less photosystem II-associated trimeric light-harvesting complexes (LHCIIb), and is null for Lhcbm1, a gene that encodes a polypeptide of LHCIIb. Photoinhibition, the decrease in photosystem II efficiency during high-light exposure, is faster in the mutant than in wild-type cells, suggesting that thermal dissipation plays a role in protecting photosystem II reaction centers from photodamage. In addition to Lhcbm1, nine other Chlamydomonas genes encoding LHCIIb polypeptides were identified in a search of the Chlamydomonas EST databases and designated Lhcbm2 to Lhcbm10. Amino acid alignments of the mature Lhcbm polypeptides demonstrated a high degree of sequence similarity with specific differences in their amino-terminal regions. Specifically, three polypeptides have threonine residues at position 3 that are predicted to be phosphorylated. Phosphorylation of light-harvesting polypeptides is critical for the redistribution of absorbed light energy between photosystem II and photosystem I. Transcript accumulation of each gene was analyzed following transfer of cells from low to high light. All ten genes exhibited the same pattern: a gradual decrease in transcript levels occurred in the first 4 hours, but levels returned to low-light levels after 24 hours of high-light.