The S4 domain is a small domain consisting of 60-65 amino acidresidues that was detected in the bacterial ribosomal proteinS4, eukaryotic ribosomal S9, two families of pseudouridinesynthases, a novel family of predicted RNA methylases, a yeastprotein containing a pseudouridine synthetase and a deaminasedomain, bacterial tyrosyl-tRNA synthetases, and a number ofuncharacterized, small proteins that may be involved intranslation regulation [1]. The S4 domain probably mediatesbinding to RNA.