A direct estimation of the context effect on the efficiency of termination.

An in vitro assay in which terminating Escherichia coli ribosomes with different stop signals in the A-site compete for a limited amount of a release factor (RF1 or RF2) has been used to estimate the relative termination efficiencies at stop codons with different adjacent downstream nucleotides. The assay allows direct measurements of relative kcat/Km parameters for the productive association of release factors to ribosomes. The kcat/Km parameter is larger for UAA(U) than for UAA(C) programmed ribosomes and the difference in kcat/Km is much larger for RF2 (about 80%) than for RF1 (about 30%). These differences in the kcat/Km parameter are not affected by the addition of release factor RF3. The only discernible effect of RF3 is a considerable acceleration of RF1/2 recycling.The estimated kcat/Km parameters correlate well with the affinities of release factors for ribosomes programmed with different stop signals. These affinities were estimated from the extent of inhibition of ribosomal recycling by high concentrations of release factors in the absence of release factor RF3. The affinity for RF2 depends on the immediate downstream context of the stop codon in the translated mRNA and is about three times higher for UAA(U) than for UAA(C). The corresponding difference in affinities for RF1 is twofold. For all stop signals studied, the estimated affinity of RF2 for terminating ribosomes is much lower than that of RF1. It is also striking that the affinity of ribosomes for a chromosomally expressed RF2 is at least three times higher than for RF2 isolated from an overproducing E. coli strain.