Ribotoxin recognition of ribosomal RNA and a proposal for the mechanism of translocation.

The ribotoxins alpha-sarcin and ricin catalyse covalent modifications in adjacent nucleotides in 28S rRNA, yet the elements of nucleic acid structure that they recognize are not only different but incompatible. This suggests that this ribosomal domain (which in two dimensions is a seven-base-pair helical stem and a 17-member single-stranded loop) has alternate conformers. Since the domain is involved in binding of aminoacyl-tRNA and GTP hydrolysis, we propose that the switch between the two configurations, perhaps initiated by the binding of elongation factors, plays a role in translocation.