This Quicktime movie shows the position of an initiator tRNA (in green) on the 70S ribosome particle. It was determined by using difference cryo-EM and comparing the structures that resulted with or without the tRNA present. The initiator tRNA is found exactly where previous work postulated it to be. You can even see the helical twist to it. More details can be found in:

Joachim Frank, Amy B. Heagle, and Rajendra K. Agrawal, "Animation of the Dynamical Events of the Elongation Cycle Based on Cryo-Electron Microscopy of Functional Complexes of the Ribosome," Journal of Structural Biology, 128:1, (1999)