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
