smFRET study of rRNA dimerization at the peptidyl transfer center.

The ribosome is a ribozyme. At the peptidyl transfer center (PTC) of 180 nt, two loops (the A- and P- loops) bind to tRNAs and position them in close proximity for efficient peptidyl ligation. There is also a 2-fold rotational symmetry in the PTC, which suggests that the precursor of the modern ribosome possibly emerged through dimerization and gene fusion. However, experiments that demonstrate the possible dimerization have not yet been published. In our investigation, we reported single molecule FRET studies of two RNA fragments that generated high FRET values. By dye-labeling the 5'-biotinylated rRNA molecules at the 3'- terminals, or labeling three different types of tRNA-like oligos, we observed that RNA scaffolds can assemble and bring several short tRNA-acceptor-domain analogs, but not full-length tRNAs, to close proximity. Mg2+ and continuous 3-way junction motifs are essential to this process, but amino acid charging to the tRNA analogs is not required. We observed RNA dimers via native gel-shifting experiments. These experiments support the possible existence of a proto-ribosome in the form of an RNA dimer or multimer.