Nucleic Acids Research, 2005, Vol. 33, No. 5 1623 Figure 2. Peptidyl transferase activity of ribosomes containing non-natural nucleoside analogs at A2451. (A) Chemical structures of all the tested nucleoside analogs. The introduced chemical modifications are depicted in red. (B) Product yields in the absence of the compensating 26mer (no oligo) or in the presence of fragments containing the wild-type sequence (wt; black bar), ribose sugar modifications (red bars) or carrying base modifications (green bars) at the position corresponding to A2451 of 23S rRNA are indicated. The yield of f-Met-puromycin formed with gapped-cp-reconstituted ribosomes carrying the wt 26mer after 240 min of incubation was taken as 1.0. Values shown represent the mean of at least three independent experiments. (C) Initial rates of peptide bond formation catalyzed by A2451-modified ribosomes. The initial rates were determined from experimental points in the linear range of the reactions within the first 30 min of incubation (with the exception of the deoxy-abasic ribosome, where the first time point that could be measured was at 240 min). The rates were normalized to the rate of reconstituted ribosomes containing the synthetic wild-type (wt) RNA fragment (A). In all cases, the background values (amount of product formed in reactions containing only native 30S subunits) were subtracted from all experimental data points.