Gene expression in chloroplasts is regulated mainly at the posttranscriptional level. In the green alga Chlamydomonas reinhardtii, synthesis of the D2 protein (PsbD), which is the rate-determining subunit for the assembly of photosystem II, depends on the RNA stability factor Nac2. In addition, the RNA binding protein RBP40 has been implicated in translational control via a U-rich element in the 5' untranslated region (5'UTR) of the psbD mRNA. Here, we report the identification of the RBP40 gene based on mass spectrometric analysis of its purified product. Unexpectedly, this was found to be identical to the previously described RNA binding protein RB38, which had been suggested to be involved in the regulation of D1 protein synthesis. However, we show that RBP40 binds to the psbD 5'UTR in a Nac2-dependent fashion both in vitro and in vivo. Molecular characterization of RBP40 RNA interference lines confirmed that RBP40 specifically affects the initiation of D2 synthesis. Native polyacrylamide gel electrophoresis, coimmunoprecipitation, and sedimentation analyses revealed that Nac2 and RBP40 form parts of a complex of 550 kD that is displaced from the psbD mRNA prior to polysome assembly. Together, these data indicate that the processes of 5'UTR-mediated RNA stabilization and translation initiation are tightly coupled in Chlamydomonas.
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