Some observations on the near ultraviolet circular dichroism of tRNA from E. coli.

The circular dichroism arising from 4-thiouridine residues in E. coli transfer RNA has been studied under conditions in which the secondary and tertiary structure of the macromolecule is either intact or totally disrupted. Studies on both unfractionated tRNA and on highly purified species of methionine-, valine-, and lysine-specific tRNA suggest that the circular dichroism of the 4-thiouridine residue is highly sensitive to its local environment in the macro-molecule and reveal interesting differences between these purified species. A new chromophore, resembling 4-thiouridine in some respects but showing distinctive chemical and optical properties, has been detected. The implications of these results on model-building studies of tRNA are discussed.