A model for the contribution of the basic region and motif X to rRNA methylation. (A) In the absence of rRNA, motif X is disordered. The electrostatic interaction of the basic region (Basic reg.) with rRNA promotes an initial encounter complex that then matures into a structure with an ordered motif X contributing to the active site of the RRAD methyltransferase. (B) In the KsgA-30S ribosome structure reported in PDB code 7v2p, helix 44 of 16S rRNA is disordered, as is the basic region and motif X of KsgA, but in C the structure reported in 7v2m helix 44 is ordered along with the basic region and motif X. The basic region is positioned to make electrostatic contacts to helix 44 of 16S rRNA.
