The sarcin/ricin loop, a modular RNA.

The conformation of a 29 base oligonucleotide called E73 has been determined in solution by NMR. E73 includes a 23 nucleotide sequence that is identical with that of a the alpha-sarcin and ricin-sensitive loop (SRL) from rat 28 S rRNA, and like the SRL in intact ribosomes, E73 is a substrate for both toxins. The SRL includes a long, conserved sequence found in the RNA of all large ribosomal subunits, which plays a critical role in the factor-dependent steps of protein synthesis. The spectroscopic observations and analysis that led to the determination of the conformation of E73 are presented. The SRL in E73 has a highly structured conformation, which is stabilized by several non-Watson-Crick base-pairs, and many properties of the SRL in the ribosome can be understood assuming that the conformation of E73 and that of the SRL in the ribosome are the same. The role of the SRL in protein synthesis is discussed in light of the conformation of E73, as is the modular relationship that exists between the conformation of the SRL and other smaller RNAs.