One protein from two open reading frames: mechanism of a 50 nt translational bypass

Translating ribosomes bypass a 50 nt coding gap in order to fuse the inform ation found in the two open reading frames (ORFs) of bacteriophage T4 gene 60. This study investigates the underlying mechanism by focusing on the com petition between initiation of bypassing and termination at the end of the first ORF. While nearly all ribosomes initiate bypassing, no more than 50% resume translation in the second ORF. Two previously described cis-acting s timulatory signals are critical for favoring initiation of bypassing over t ermination. Genetic analysis of these signals supports a working model in w hich the first (a stem-loop structure at the junction between the first ORF and the coding gap) interferes with decoding in the A-site, and the second (a stretch of amino acids in the nascent peptide encoded by the first ORF) destabilizes peptidyl-tRNA-mRNA pairing.