On the role of rRNA tertiary structure in recognition of ribosomal protein L11 and thiostrepton.

Ribosomal protein L11 and an antibiotic, thiostrepton, bind to the same highly conserved region of large subunit ribosomal RNA and stabilize a set of NH4(+)-dependent tertiary interactions within the domain. In vitro selection from partially randomized pools of RNA sequences has been used to ask what aspects of RNA structure are recognized by the ligands. L11-selected RNAs showed little sequence variation over the entire 70 nucleotide randomized region, while thiostrepton required a slightly smaller 58 nucleotide domain. All the selected mutations preserved or stabilized the known secondary and tertiary structure of the RNA. L11-selected RNAs from a pool mutagenized only around a junction structure yielded a very different consensus sequence, in which the RNA tertiary structure was substantially destabilized and L11 binding was no longer dependent on NH4+. We propose that L11 can bind the RNA in two different 'modes', depending on the presence or absence of the NH4(+)-dependent tertiary structure, while thiostrepton can only recognize the RNA tertiary structure. The different RNA recognition mechanisms for the two ligands may be relevant to their different effects on protein synthesis.