Aa. Szewczak et al., THE CONFORMATION OF THE SARCIN RICIN LOOP FROM 28S RIBOSOMAL-RNA, Proceedings of the National Academy of Sciences of the United Statesof America, 90(20), 1993, pp. 9581-9585
The sarcin/ricin loop is a highly conserved sequence found in the RNA
of all large ribosomal subunits. The cytotoxins alpha-sarcin and ricin
both inactivate ribosomes by cleaving a single bond in that loop. Onc
e it has been attacked, ribosomes no longer interact with elongation f
actors properly, and translation stops. We have determined the conform
ation of the sarcin/ricin loop by multinuclear NMR spectroscopy using
E73, a 29-nucleotide RNA that has the sarcin/ricin loop sequence and t
hat is sensitive to both toxins in vitro. The sarcin/ricin loop has a
compact structure that contains several purine.purine base pairs, a GA
GA tetraloop, and a bulged guanosine adjacent to a reverse Hoogsteen A
.U pair. It is stabilized by an unusual set of cross-strand base-stack
ing interactions and imino proton to phosphate oxygen hydrogen bonds.
In addition to having interesting structural features, this model expl
ains many of the biochemical observations made about the loop's struct
ure and its reactivity with cytotoxins, and it sheds light on the loop
's interactions with elongation factors.