Localization of 5′ and 3′ ends of the ribosome-bound segment of template polynucleotides by immune electron microscopy.

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Abstract
Poly(U) with an average chain length of 40‐70 nucleotides was modified at the 5′‐ or 3′‐terminal residues with 2,4‐dinitrophenyl derivatives. The modified poly(U) was used to form 30S.poly(U) or 70S.poly(U).Phe‐tRNA complexes. Localization of the 5′ and 3′ ends of the template polynucleotide on the 30S subunit and the 70S ribosome was performed by immune electron microscopy using antibodies against dinitrophenyl haptens. The 5′ and 3′ ends of poly(U) (putative entry and exit sites of the message) were found in the same region both on the 30S subunit and the 70S ribosome. They were located on the dorsal side of the 30S subunit between the head and the body near the groove bordering the side ledge (platform). Comparison of the size of this region with the possible length of the polynucleotide chain covered by the ribosome allowed us to suggest that the message makes a ‘U‐turn” (or forms a ‘loop’) as it passes through the ribosome.