The in vivo frameshift assays were performed as described previously (25) . Briefly, 1 ml of an exponentially growing culture was induced with Isopropyl b-D-Thiogalactopyranoside (IPTG) to a final concentration of 1 mM at an optical density of 0.4-0.7 measured at 436 nm (OD 436 ). After induction for 15 min, the culture was pulse-labelled with $10 mCi L-[ 35 S]-methionine for 20 s and chased with 100 mg L-methionine for 2 min before being transferred to 25 ml of chloramphenicol (100 mg/ml) on ice. The cells were harvested by centrifugation and proteins were boiled in SDS buffer and separated by 9% SDS-PAGE. The gel was dried and placed on a phosphor imager screen (Molecular Dynamics) and left to expose for 1-3 days. Relative amount of protein of the relevant polypeptides was quantified using ImageQuant software and the frameshift efficiency (e) was determined as follows: