YxiN is a modular protein combining a DEx^D/_H core and a specific RNA-binding domain

DEx^D/_H proteins, typically described as RNA helicases, participate in rearrangement of RNA-RNA and possibly RNA-protein complexes in the cell. Aside from the conserved DEx^D/_H core, members of this protein family often contain N- and C-terminal extensions that are responsible for additional functions. The Bacillus subtilis DEx ^D/_H-box protein YxiN and its Escherichia coli ortholog DbpA contain an ∼80 amino acid C-terminal extension that has been proposed to specifically interact with a region of 23 S ribosomal RNA including hairpin 92. In this study, the DEx^D/_H-box core and the C-terminal domain of YxiN were expressed and characterized as separate proteins. The isolated DEx^D/_H-box core, YxCat, had weak, nonspecific RNA binding activity and showed RNA-stimulated ATPase activity with a K _m(ATP) that resembled several non-specific DEx^D/ _H proteins. The isolated C-terminal domain, YxRBD, bound RNA with the high affinity and specificity seen with full-length YxiN. Thus, YxiN is a modular protein combining the activities of the YxCat and YxRBD domains. Footprinting of YxiN and YxRBD on a 172-nucleotide fragment of 23 S rRNA was used to identify the sites of interaction of the C-terminal and helicase domains with the RNA.