The S4 domain is approximately 60-65 amino acid residues long, occurs in a single copy at various positions in different proteins and was originally found in pseudouridine syntheses, a bacterial ribosome-associated protein.

In this way proteins serve to organise and stabilise the rRNA tertiary structure.

In addition, the S4 domain binds a complex pseudoknot and represses translation.

[1] The S4 protein domain is composed of three alpha helices and five beta strands.

It is organized as an antiparallel sheet in a Greek key motif.