Different conformations of tRNA in the ribosomal P‐site and A‐site

Footprinting studies involving radioactively end‐labelled tRNA species bound at either the ribosomal P‐or A‐site have yielded information that the tRNA's conformation is different in the two sites. Appropriate controls showed the relevance of using poly(U)‐directed tRNAPhe binding in the P‐site and Phe‐tRNAPhe in the A‐site. Digestion of the tRNA species was effected by RNases T1, T2 and cobra venom RNase. Experiments were performed with tRNAs 32P‐labelled at either end to establish positions of primary cuts more confidently. In addition to the common protection of the aminoacyl‐stem and anticodon‐arm, footprinting experiments revealed striking differences in the accessibility of the T‐and d‐loops of tRNAs bound in the P‐and A‐sites. We observed a more open structure for the tRNA in the A‐site. These results are consistent with a dynamic structure of tRNA during the translocation step of protein biosynthesis.