A major focus of our research is the application of photochemical techniques, the study of the structure of biological macromolecules and macromolecular assemblies. Of particular current interest is the bacterial ribosome where we are seeking to elucidate the topography of the transfer RNA (tRNA) binding sites. In our approach photoreactive nucleosides are site-specifically substituted for natural nucleosides in a tRNA molecule—2-azidoadenosine for the adenosine of position 76 in tRNAPhe, for example. The modified tRNAs are then bound to the ribosome and irradiated with UV light, thereby establishing covalent crosslinks between the photoreactive nucleoside and nearby protein and/or RNA components of the ribosome. The sites of crosslinking within the ribosome are then identified.