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 oligodeoxyribonu cleotide probe and its exploitation in identifying 50S ribosomal subun it components neighboring the alpha-sarcin loop, The probe is compleme ntary to 23S rRNA nt 2653-2674, Photolysis of the complex formed betwe en the probe and 50S subunits leads to site-specific probe photoincorp oration into proteins L2, the most highly labeled protein, L1, L15, L1 6 and L27, labeled to intermediate extents, and L5, L9, L17 and L24, e ach labeled to a minor extent, Portions of each of these proteins thus lie within 23 Angstrom of nt U2653, These results lead us to conclude that the alpha-sarcin loop is located at the base of the L1 projectio n within the 50S subunit. Such placement, near the peptidyl transferas e center, provides a rationale for the extreme sensitivity of ribosoma l function to cleavage of the alpha-sarcin loop.