Evolution of the large-subunit ribosomal RNA binding site for protein L23/25

The region of the large-subunit rRNA encompassing the D7 divergent domain is organized within eukaryotes in a patchwork of short conservative secondary-structure features interspersed with more rapidly evolving sequences. It contains the attachment site of protein L25 (E. coli L23), which binds rRNA in the first stages of ribosome assembly, suggesting a crucial importance of this region in ribosome elaboration and functioning. A better understanding of its roles requires a good knowledge of its mode of structural variation during the course of evolution. With this aim, we sequenced the D7 region for 24 new invertebrate species belonging to annelids, molluscs, arthropods, and eight other deep-branching invertebrate phyla. Their comparison allowed us to propose refinements in previous eukaryotic folding models. A detailed analysis of the pattern of variation at each position both within the D7 region and along the L23/25 sequence by reference to previous heterologous binding experiments gives new insight into the rRNA-protein contacts. We identified in the D7 region and L23/25, respectively, six and five positions presenting a pattern of variation compatible with experimental results, three of which show coincident variations which support their possible involvement in the rRNA-L23/25 binding.