Non-Watson-Crick base pairs or mismatches in RNA crystal structures have be
en summarized and their effects on the geometry of RNA molecules and their
biological implications have been discussed. The mismatches can distort the
helical structures by producing kinks/bends and can increase or decrease t
he major groove width, depending on the sequence. The helical irregularitie
s may serve as potential sites for interaction with proteins. The different
base-stacking patterns of the mismatches together with the flanking Watson
-Crick base pairs can also provide unique recognizable surfaces for interac
tion with ligands. Mismatches are usually more hydrated than the Watson-Cri
ck base pairs. Details of hydration of mismatches show the interaction of w
ater molecules with the appendant functional groups of the mismatches. Mism
atches with only one base-base hydrogen bond have been observed, in which b
ridging water molecules replace-one of the direct base pair hydrogen bonds
to stabilize the conformation. (C) 1999 John Wiley & Sons, Inc.