L9 N-domain-like superfamily

The N-terminal domain of the ribosomal protein L9 is a regulatory RNA-binding module that binds to 23rRNA. L9 is composed of two domains and functions as a structural protein in the large subunit of the ribosome.

The N-terminal domain of eukaryotic RNase HI, which is lacking in retroviral and prokaryotic enzymes, shows a striking structural similarity to the L9 N-terminal domain, and may also function as a regulatory RNA-binding module. Eukaryotic RNases HI possess either one or two copies of the small N-terminal domain, in addition to the well-conserved catalytic RNase H domain. RNase HI belongs to the family of ribonuclease H enzymes that recognise RNA:DNA hybrids and degrade the RNA component.

The structures of both the L9 [8306963] and the RNase HI [10448044] N-terminal domains consist of a three-stranded antiparallel beta-sheet sandwiched between two short alpha-helices. The hydrophobic core of the domain is formed by the conserved residues that are involved in the packing of the alpha-helices onto the beta-sheet. The (beta)2/alpha/beta/alpha topology of the domain differs from the structures of known RNA binding domains such as the double-stranded RNA binding domain (dsRBD), the hnRNP K homology (KH) domain and the RNP motif.