Ribosomal proteins S7 and L1 are located close to the decoding site ofE.coliribosome — affinity labeling studies with modified tRNAs carrying photoreactive probes attached adjacent to the 3′-end of the anticodon

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Abstract
Two photoreactive azindonitrophenyl probes have been attached to Yeast methionine elongator tRNA by chemical modification of the N6-(threoninocarbonyl)adenosine located next to the 3''-end of the anticodon. The maximum distance between the purine ring and the azido group estimated for the two probes is 16-17 and 23-24.ANG., respectively. Binding and cross-linking of the uncharged, modified tRNAs to E. coli ribosomes have been studied with and without poly(A,U,G) as a message, under conditions directing uncharged tRNAs preferentially to the P-site. The modified tRNAs retain their binding activity and upon irradiation bind covalently to the ribosome with very high yields. Protein S7 is the major cross-linking target for both modified tRNAs, in the presence or absence of poly(A,U,G). Protein L1 and to a lesser extent proteins L33 and L27 have been found to be cross-linked with the short probe. Cross-linking to 16S rRNA reaches significant levels only in the absence of the message.