7 Transfer RNA: An RNA for All Seasons

Long before the existence of an RNA world was accepted, Francis Crick attributed to tRNA enzymatic properties (like a protein) because of its versatility and ability to recognize so many proteins with exquisite specificity. Presumably, this was meant to reflect a well-known property of proteins: conformational flexibility, which is a prerequisite for the manifold recognition characteristics of this small RNA molecule. Even though tRNA alone has been shown to have only marginal enzymatic properties (as discussed below), the many roles of tRNA demand that this molecule assume different conformations to provide diverse points of contact for the selective recognition of many different proteins. Over the years, tRNA has provided complex challenges in different experimental areas. For biophysicists it provided the first high-resolution X-ray crystal structure of an RNA (Kim et al. 1974; Robertus et al. 1974) and of an RNA:protein complex (Rould et al. 1989), whereas it chal-lenged biochemists to achieve the first sequence determination of a