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A component of the photosystem H reaction center, the 32-kDa protein, is rapidly turned over in the light. The mechani of its light-dependent metabolism is largely unknown. We quantified the rate of 32-kDa protein degradation over a broad spectral range (UV, visible, and far red). The quantum yield for degradation was highest in the UVB am) region. Spectral evidence demonstrates two distincty different photosensitizers for 32-kDa protein degradation. The data implicate the bulk photosynthetic pigments (primarily chlorophyll) in the visible and far red ens, and plastoquinone (in one or more of its redox states) in the UV region. A signiicant portion of 32-kDa protein degradation in sunlight is attributed to UVB irradiance.Photosynthetic oxygen evolution is catalyzed by photosystem II (PSII), a complex of several proteins and pigments located in the chloroplast membrane (1, 2). The PSII reaction center contains three proteins-the 32-kDa protein, D2, and cytochrome b559-as well as bound chlorophylls, pheophytins, quinones, and nonheme iron (3, 4). The 32-kDa protein (also known as D1 and the QB protein) has received considerable attention because it is a major product of the chloroplast protein synthesizing machinery (5, 6), is rapidly turned over as a function of the visible light intensity (7), and is the direct target for PSII herbicides such as atrazine and diuron (8, 9). These herbicides block electron transport through PSII (10) and inhibit 32-kDa protein degradation (7) by displacing a liganded quinone (QB) from the protein (11).Indeed, the primary site of 32-kDa protein cleavage (12) maps to a phylogenetically conserved domain (5), which is adjacent to the quinone and PSII herbicide binding pocket (13). The cleavage site is contiguous with an a-helix destabilizing region (12) common to many rapidly degraded proteins (14).Although Other Methods. Conditions for in vivo radiolabeling, isolation of membrane proteins, their fractionation by SDS/ PAGE, quantification by densitometry, and determination of in vivo absorptance (1 -transmittance) and photosynthetic activity spectra are described in the figure legends. RESULTS AND DISCUSSIONTurnover of 32-kDa Protein in UV Light. The 32-kDa protein is rapidly turned over in visible light (400-700 nm), where both PSII and photosystem I (PSI) are functional (7), and in far red light (>700 nm), where PSI is predominantly active (15). The results presented in Fig. lA

The 32 kd photosystem II protein of plant chloroplasts is rapidly turned over in the light. The initial events in the degradation of the 32 kd protein were studied. A 23.5 kd breakdown product was identified in Spirodela oligorrhiza membranes using immunological analysis. The 23.5 kd polypeptide was shown to be derived from the amino‐terminal portion of the 32 kd protein using partial proteolytic fingerprinting. An in vivo precursor–product relationship between the 32 kd protein and the 23.5 kd polypeptide was kinetically demonstrated by radiolabeling and pulse‐chase experiments. The cleavage site yielding the 23.5 kd polypeptide was localized to a functionally active region (between helices IV and V) of the 32 kd protein. We propose that an alpha‐helix‐destabilizing ‘degradation’ sequence, bordered by arginine residues 225 and 238, is involved in the formation of the 23.5 kd polypeptide.

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Mixed infections of cucurbits by Cucumber mosaic virus (CMV) and potyviruses exhibit a synergistic interaction. Zucchini squash and melon plants coinfected by the potyvirus Zucchini yellow mosaic virus (ZYMV) and either Fny-CMV (subgroup IA) or LS-CMV (subgroup II) displayed strong synergistic pathological responses, eventually progressing to vascular wilt and plant death. Accumulation of Fny- or LS-CMV RNAs in a mixed infection with ZYMV in zucchini squash was slightly higher than infection with CMV strains alone. There was an increase in CMV (+) strand RNA levels, but no increase in CMV (-) RNA3 levels during mixed infection with ZYMV. Moreover, only the level of capsid protein from LS-CMV increased in mixed infection. ZYMV accumulated to similar levels in singly and mixed infected zucchini squash and melon plants. Coinfection of squash with the potyvirus Watermelon mosaic virus (WMV) and CMV strains increased both the Fny-CMV RNA levels and the LS-CMV RNA levels. However, CMV (-) strand RNA3 levels were increased little or not at all for CMV on coinfection with WMV. Infection of CMV strains (LS and Fny) containing satellite RNAs (WL47-sat RNA and B5*-sat RNA) reduced the accumulation of the helper virus RNA, except when B5*-sat RNA was mixed with LS- CMV. However, mixed infection containing ZYMV and the CMV strains with satellites reversed the suppression effect of satellite RNAs on helper virus accumulation and increased satellite RNA accumulation. The synergistic interaction between CMV and potyviruses in cucurbits exhibited different features from that documented in tobacco, indicating there are differences in the mechanisms of potyvirus synergistic phenomena.