While both À1 and +1 PRF are kinetically driven events, the substrates for the slippage are distinct: À1 PRF requires that both the ribosomal A-and P-sites are occupied by tRNAs, while+1 PRF occurs while the A-site is empty (38) . Rates of +1 PRF were monitored using a cis-acting signal derived from the Ty1 retrotransposable element using pYDL-Ty1. Baseline +1 PRF efficiencies in cells expressing wild-type RPL11B were 10.98% ± 0.30%. 51-4A had no effects on +1 PRF, while 54-7A promoted a small but statistically significant increase (1.18 ± 0.06-fold of wild type; Figure 3 ). Significant changes in +1 PRF were also observed in the Y52A, Y52S, Y52N, Y52E, Y52H and Y52F mutants. mRNA decoding occurs in the small subunit decoding center, and changes in termination codon recognition (nonsense suppression) is another indicator of altered translational fidelity. pYDL-UAA (39) , which contains an in-frame termination codon immediately 5 0 of the firefly luciferase gene, was used to monitor this parameter. The baseline rate of nonsense suppression in cells expressing RPL11B was 0.137% ± 0.003%. The 51-4A mutant slightly improved this aspect of translational fidelity, with nonsense suppression levels decreasing to 0.88 ± 0.04-fold of wild-type levels. 54-7A did not affect UAA recognition (Figure 3) . Y52Á, Y52A, Y52S, Y52N, Y52E and Y52H all promoted increased rates of nonsense suppression ranging from 1.31-to 1.78-fold wild type. pYDL-AGC 218 tests missense suppression levels by monitoring rates of incorporation of an arginine (AGA) near-cognate amino acid instead of a cognate serine (AGC) at the catalytic codon 218 within the firefly luciferase gene as previously described (24) . Thus, in this assay, mis-utilization of near-cognate tRNA Arg at the Ser AGC codon restores firefly luciferase activity. Wild-type missense levels were measured at 0.074% ± 0.002, comparable to previous studies (24) . Mutant 51-4A had significantly higher levels of missense suppression (measured at 1.21 ± 0.03-fold wild-type), while 54-7A did not significantly affect this phenomenon (1.07 ± 0.04 fold wild type) ( Figure 3 ). Missense suppression was not assayed for the single amino acid mutants.