Topography of the E site on the Escherichia coli ribosome.

Three photoreactive tRNA probes have been utilized in order to identify ribosomal components that are in contact with the aminoacyl acceptor end and the anticodon loop of tRNA bound to the E site of Escherichia coli ribosomes. Two of the probes were derivatives of E. coli tRNA(Phe) in which adenosines at positions 73 and 76 were replaced by 2-azidoadenosine. The third probe was derived from yeast tRNA(Phe) by substituting wyosine at position 37 with 2-azidoadenosine. Despite the modifications, all of the photoreactive tRNA species were able to bind to the E site of E. coli ribosomes programmed with poly(A) and, upon irradiation, formed covalent adducts with the ribosomal subunits. The tRNA(Phe) probes modified at or near the 3' terminus exclusively labeled protein L33 in the 50S subunit. The tRNA(Phe) derivative containing 2-azidoadenosine within the anticodon loop became cross-linked to protein S11 as well as to a segment of the 16S rRNA encompassing the 3'-terminal 30 nucleotides. We have located the two extremities of the E site-bound tRNA on the ribosomal subunits according to the positions of L33, S11 and the 3' end of 16S rRNA defined by immune electron microscopy. Our results demonstrate conclusively that the E site is topographically distinct from either the P site or the A site, and that it is located alongside the P site as expected for the tRNA exit site.