Localization of an oligodeoxynucleotide complementing 16S ribosomal RNA residues 520–531 on the small subunit ofEscherichia coilribosomes: electron microscopy of ribosome-cDNA-antibody complexes

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Abstract
The oligodeoxynucleotide dACCGCGGCTGCT, complementary to Escherichia coli small ribosomal subunit RNA residues 520-531, has been used to probe subunit conformation and to localize the sequence in the subunit. Conditions for binding of the cDNA to 30S subunits were optimized and specificity in the interaction was demonstrated by RNase H cleavage. Three kinds of terminal modification of this cDNA were used to allow its localization by immune electron microscopy. A solid phase support with 5''-dimethoxytrity-N6-.DELTA.2-isopentenyl-adenosine linked to controlled pore glass was synthesized, and used to prepare oligomer with an added 3''-terminal residue of isopentenyl adenosine. cDNA with a 5'' primary amine substituent was modified with 1-fluoro-2,4-dinitrobenzene to prepare 5''-dinitrophenyl oligonucleotide, and both modifications together gave doubly-derivatized probes. Immune electron microscopy with antibodies to dinitrophenol, isopentenyl adenosine, or both, was used to place the cDNA on 30S subunits. In each case the probe was placed at a single site at the junction on the head and body of the subunit, near the decoding site and the area in which elongation factor Tu is bound. It is proposed that this segment of ribosomal RNA functions in mRNA binding and orientation.