Characterization and RNA-binding properties of a chloroplast S1-like ribosomal protein

Control of translation is an important step in chloroplast gene expression. A first control can be exerted during the initiation complex formation which, in Escherichia coli, involves the ribosomal protein (r-protein) S1. A cDNA clone have been characterized which codes for the precursor of the chloroplast r-protein CS1. The mature protein consists of a central core which shows 31.5% amino acid homology to the E. coli protein S1. The CS1 is considerably shorter (40 kDa) than the protein S1 (61 kDa). The core fragment contains three degenerated repeats which show homology to both the ribosome- and the RNA-binding domain of S1. RNA-protein CS1 interactions were studied by UV cross-linking and toe-printing. CS1 has been over-expressed in E. coli, and after purification its RNA-binding properties were studied in vitro. We conclude that the CS1 exhibits an RNA-binding activity which is actively involved in the chloroplast initiation complex formation. It is shown that the CS1 binds to poly(A) in contrast with S1 which binds strongly to poly(U). These results are interpreted in relation to the presence of poly(A)-rich regions in chloroplast transcripts of higher plants.