uS4 is a cytosol-exposed protein of the small ribosomal subunit with the most highly-diverged structure between bacterial and eukaryotic species. Both in bacteria and eukaryotes, this protein is about 200 residues long, whereas its conserved core comprises only ~50 amino acid residues (1).
In bacteria uS4 has bacteria-specific N-terminal extension that forms part of the mRNA entry site (2). This extension is also considered to serve as a ribosome-built mRNA helicase that is capable to unwind short stem-loop structures (3). In eukaryotes, this extension is replaced by a eukaryote-specific ribosomal protein eS30.