<p> The K homology domain is a common RNA-binding motif present in one or multiple copies in both prokaryotic and eukaryotic regulatory proteins. The KH motifs may act cooperatively to bind RNA in the case of multiple motifs, or independently in the case of single KH motif proteins. Prokaryotic (pKH) and eukaryotic (eKH) KH domains share a KH-motif, but have different topologies. The pKH domain has been found in a number of proteins, including the N-terminal domain of the S3 ribosomal protein [<cite idref="PUB00011737"/>], the C-terminal domain of Era GTPase [<cite idref="PUB00011738"/>] and the two C-terminal domains of the NusA transcription factor [<cite idref="PUB00011739"/>]. The structure of the pKH domain consists of a two-layer alpha/beta fold in the arrangement alpha/beta(2)/alpha/beta. </p><p>More information about these proteins can be found at Protein of the Month: RNA Exosomes [<cite idref="PUB00035573"/>].</p>