Active Translation of the D-1 Protein of Photosystem II in Senescing Leaves

Abstract

ABSTRACTin gene expression. In particular, there is a selective depletion reflecting curtailed synthesis of Cytsfand b6 from the thylakoids of senescing leaves (3,25). This correlates with independent biochemical measurements showing that transport of electrons through the Cyt f/b6 complex is the rate-limiting step accounting for the decline in noncyclic electron transport (12,14). Indeed, the decline in noncyclic electron transport with advancing senescence is much more marked than the decline in the independent activities of PSI and PSII (14,22).Changes in thylakoid protein synthesis with advancing senescence have also been demonstrated by pulse labeling with [35S]methionine. Of particular interest is the finding that, whereas synthesis of the majority of thylakoid-associated photosynthetic proteins, including the 68-kD apoprotein of PSI, the a and : subunits of ATPase, light-harvesting Chl binding protein, and Cytsfand b6, decline by two-and fourfold during senescence, there is no decrease in synthesis of the D-1 protein of PSII (25). This does not, however, result in significant changes in steady-state levels of these proteins in the thylakoids of senescent leaves (25), presumably because, with the exception of D-1, thylakoid proteins generally have long halflives. There are also senescence-related changes in transcript levels for these proteins, although the differential changes in protein synthesis cannot be fully accounted for by alteration in steady-state message levels (1, 2).The finding that D-1 continues to be strongly synthesized in senescing leaves well after a sharp decline in the synthesis of other thylakoid photosynthetic proteins may be related to its comparatively short half-life and the need to maintain functional D-1 even under conditions of greatly reduced photosynthetic electron transport. The turnover of D-1 is very rapid, light-dependent, and inhibited by herbicides such as DCMU and atrazine that displace the second electron-accepting plastoquinone of PSII (20,28). This rapid turnover has been correlated with the presence of a 14-amino acid a helix in the protein, termed a destabilizing sequence, that is located adjacent to the putative cleavage domain (1 1). Recent data obtained with radical scavengers suggest that light-dependent D-1 degradation is initiated by activated oxygen, likely the hydroxyl radical, which causes it to undergo a change in conformation (26).Previous pulse/chase labeling studies have indicated that the turnover rate for D-1 does not change with advancing senscence (25), which suggests that the capability for generating activated oxygen may be retained in the senescing chloroplast notwithstanding a decline by more than 40% in 589 www.plantphysiol.org on May 9, 2018 -Published by Downloaded from

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