Recognition of a complex substrate by the KsgA/Dim1 family of enzymes has been conserved throughout evolution.

Ribosome biogenesis is a complicated process, involving numerous cleavage, base modification and assembly steps. All ribosomes share the same general architecture, with small and large subunits made up of roughly similar rRNA species and a variety of ribosomal proteins. However, the fundamental assembly process differs significantly between eukaryotes and eubacteria, not only in distribution and mechanism of modifications but also in organization of assembly steps. Despite these differences, members of the KsgA/Dim1 methyltransferase family and their resultant modification of small-subunit rRNA are found throughout evolution and therefore were present in the last common ancestor. In this paper we report that KsgA orthologs from archaeabacteria and eukaryotes are able to complement for KsgA function in bacteria, both in vivo and in vitro. This indicates that all of these enzymes can recognize a common ribosomal substrate, and that the recognition elements must be largely unchanged since the evolutionary split between the three domains of life.