Abstract
Photosystem II activities as well as polypeptide levels were compared in mesophyll and bundle-sheath chloroplasts of the two monocotyledonous NADP-malic enzyme-type C4 species Zea may L. (maize) and Sorghum bicolor (L.) Moench. Small decreases in PSII activities as well as protein levels were observed when comparing maize seedlings of varying ages. In contrast, PSII activity was not noticeable in the bundle-sheath chloroplasts of even very young Sorghum seedlings, whereas substantial amounts of some of the PSII polypeptides analyzed were still detectable. In both species the degree of reduction was found to vary drastically with respect to the different PSII subunits. The greatest depletion was observed for the 23- and 16-kDa regulatory polypeptides of the water-splitting apparatus and the 10-kDa polypeptide (psbR product), indicating that PSII activity in bundle-sheath chloroplasts is limited due to a depletion at the oxidizing site of this complex. To gain insight into the regulatory principles which control the cell-specific expression of the plastome-encoded PSII genes an in-organello translation system with intact mesophyll and bundle-sheath chloroplasts of maize was established. The synthesis of the D1 polypeptide of the PSII reaction center was found to be reduced in bundle-sheath chloroplasts, reflecting the de creased levels of Dl in this type of chloroplast.
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