New photoreactive tRNA derivatives for probing the peptidyl transferase center of the ribosome

Sucrose-gradient profiles of dissociated ribosomal subunits after cross-linking. (A) unmodified tRNA^Phe, (B) (2N₃A76)tRNA^Phe, (C) (2N₃dA76)tRNA^Phe, (D) (2N₃2′OMeA76)tRNA^Phe, and (E) (2N₃dA76)]tRNA^Phe-p-Puro. Both UV absorbance (solid line) and radioactivity (dashed line with data points) were monitored as the gradients were fractionated. With the exception of unmodified tRNA^Phe, the ligands cross-linked exclusively to the 50S subunit, as shown by the comigration of radioactivity with the 50S peak.