Placement of the alpha-sarcin loop within the 50S subunit: evidence derived using a photolabile oligodeoxynucleotide probe.

We report the synthesis of a radioactive, photolabile oligodeoxyribonucleotide probe and its exploitation in identifying 50S ribosomal subunit components neighboring the alpha-sarcin loop. The probe is complementary to 23S rRNA nt 2653-2674. Photolysis of the complex formed between the probe and 50S subunits leads to site-specific probe photoincorporation into proteins L2, the most highly labeled protein, L1, L15, L16 and L27, labeled to intermediate extents, and L5, L9, L17 and L24, each labeled to a minor extent. Portions of each of these proteins thus lie within 23 A of nt U2653. These results lead us to conclude that the alpha-sarcin loop is located at the base of the L1 projection within the 50S subunit. Such placement, near the peptidyl transferase center, provides a rationale for the extreme sensitivity of ribosomal function to cleavage of the alpha-sarcin loop.