Ribosomes bypass a 50 nucleotide non-coding segment of mRNA between the two open 
reading frames of bacteriophage T4 gene 60 in order to synthesize a 
topoisomerase subunit. While nearly all ribosomes appear to initiate bypassing, 
only 50 % resume translation in the second open reading frame. Failure to bypass 
is shown here to be independent of the stop codon at the end of the first open 
reading frame and to be amplified by mutant variants of tRNA(Gly)(2) known to 
diminish bypassing efficiency. Unproductive bypassing may result from premature 
dissociation of peptidyl-tRNAs from ribosomes (drop-off) or resumption of 
translation at inappropriate sites. Assessment of the influence of factors known 
to induce drop-off reveals that ribosome recycling factor accounts for a small 
fraction of unproductive bypassing products, but none of the other known factors 
appear to play a significant role. Resumption of translation at inappropriate 
sites appears to be minimal, which suggests that spontaneous release of the 
peptidyl-tRNA may account for the remaining unproductive bypassing products and 
may be inherent to the gene 60 bypassing mechanism.