This family is expressed by almost all bacterial and eukaryotic genomes but not by archaea. Its function is to down-regulate protein synthesis under conditions of nutrient shortage, and it does this by binding to protein L14 of the large ribosomal subunit, thus acting as a ribosomal silencing factor (RsfS) by blocking the joining of the ribosomal subunits [1]. This family is structurally homologous to nucleotidyltransferases.

1: RsfA (YbeB) proteins are conserved ribosomal silencing factors. Hauser R, Pech M, Kijek J, Yamamoto H, Titz B, Naeve F, Tovchigrechko A, Yamamoto K, Szaflarski W, Takeuchi N, Stellberger T, Diefenbacher ME, Nierhaus KH, Uetz P; PLoS Genet. 2012;8:e1002815. PMID:22829778