The S4 domain is a small domain consisting of 60-65 amino acid residues that was detected in the bacterial ribosomal protein S4, eukaryotic ribosomal S9, two families of pseudouridine synthases, a novel family of predicted RNA methylases, a yeast protein containing a pseudouridine synthetase and a deaminase domain, bacterial tyrosyl-tRNA synthetases, and a number of uncharacterized, small proteins that may be involved in translation regulation [1]. The S4 domain probably mediates binding to RNA.

1: Novel predicted RNA-binding domains associated with the translation machinery. Aravind L, Koonin EV; J Mol Evol 1999;48:291-302. PMID:10093218

2: The crystal structure of ribosomal protein S4 reveals a two-domain molecule with an extensive RNA-binding surface: one domain shows structural homology to the ETS DNA-binding motif. Davies C, Gerstner RB, Draper DE, Ramakrishnan V, White SW; EMBO J 1998;17:4545-4558. PMID:9707415