title: The role of chloroplast-encoded protein biosynthesis on the rate of D1 protein degradation in Dunaliella salina.
abstract: Mechanistic aspects of the Photosystem II (PS II) damage and repair cycle in Dunaliella salina were investigated. The work addressed the role of chloroplast-encoded protein biosynthesis on the rate of the D1 protein (chloroplast psbA gene product) degradation, following photoinhibition of PS II under in vivo conditions. Cells were grown under different light-intensities and the rate of D1 photodamage and degradation was measured via pulse-chase measurements with ((35)S)sulfate. It is shown that no detectable difference exists in the rate of D1 degradation in D. salina, measured in the presence or absence of lincomycin, a chloroplast protein biosynthesis inhibitor. The results suggest that de novo D1 biosynthesis does not play a role in the regulation of D1 degradation. In low-light (100 μmol photons m(-2) s(-1)) grown cells, the rate of photodamage to D1 did not exceed the rate of its degradation and replacement. In high-light (2200 μmol photons m(-1) s(-1)) grown cells, the rate of D1 photodamage was faster than the rate of its degradation, resulting in a significant accumulation of photoinactivated PS II centers in the chloroplast thylakoids (chronic photoinhibition). The latter was coincident with the appearance of a 160 kD complex that contained photodamaged D1. Electron micrographs of D. salina thylakoids revealed extensive grana stacks in the thylakoid membrane of low-light grown cells. Only rudimentary appressions consisting of simple membrane pairings were found in the high-light grown cells. The results are discussed in terms of the regulation of D1 degradation in chloroplasts under in vivo conditions.
authors.affiliation: Department of Plant Biology, University of California, 411 Koshland Hall, 94720-3102, Berkeley, CA, USA.
authors.forename: C
authors.initials: C
authors.lastname: Vasilikiotis
authors.forename: A
authors.initials: A
authors.lastname: Melis