A chloroplast RNA binding protein from stromal thylakoid membranes specifically binds to the 5' untranslated region of the psbA mRNA.

The intrachloroplastic localization of post-transcriptional gene expression steps represents one key determinant for the regulation of chloroplast development. We have characterized an RNA binding protein of 63 kDa (RBP63) from Chlamydomonas reinhardtii chloroplasts, which cofractionates with stromal thylakoid membranes. Solubility properties suggest that RBP63 is a peripheral membrane protein. Among RNA probes from different 5' untranslated regions of chloroplast transcripts, RBP63 preferentially binds to the psbA leader. This binding is dependent on a region comprising seven consecutive A residues, which is required for D1 protein synthesis. A possible role for this newly discovered RNA binding protein in membrane targeting of psbA gene expression is discussed.