Escherichia coli YhbY is associated with pre-50S ribosomal subunits, which implies a function in ribosome assembly. GFP fused to a single-domain CRM protein from maize localizes to the nucleolus, suggesting that an analogous activity may have been retained in plants. A CRM domain containing protein in plant chloroplasts has been shown to function in group I and II intron splicing. In vitro experiments with an isolated maize CRM domain have shown it to have RNA binding activity. These and other results suggest that the CRM domain evolved in the context of ribosome function prior to the divergence of Archaea and Bacteria, that this function has been maintained in extant prokaryotes, and that the domain was recruited to serve as an RNA binding module during the evolution of plant genomes. YhbY has a fold similar to that of the C-terminal domain of translation initiation factor 3 (IF3C), which binds to 16S rRNA in the 30S ribosome.