IS THE IN-FRAME TERMINATION SIGNAL OF THE ESCHERICHIA-COLI RELEASE FACTOR-II FRAMESHIFT SITE WEAKENED BY A PARTICULARLY POOR CONTEXT

The synthesis of release factor-2 (RF-2) in bacteria is regulated by a high efficiency +1 frameshifting event at an in-frame UGA stop codon, The stop codon does not specify the termination of synthesis efficien tly because of several upstream stimulators for frameshifting, This st udy focusses on whether the particular context of the stop codon withi n the frameshift site of the Escherichia coli RF-2 mRNA contributes to the poor efficiency of termination, The context of UGA in this recodi ng site is rare at natural termination sites in E.coli genes, We have evaluated how the three nucleotides downstream from the stop codon (+4 , +5 and +6 positions) in the native UGACUA sequence affect the compet itiveness of the termination codon against the frameshifting event, Ch anging the C in the +4 position and, separately, the A in the +6 posit ion significantly increased the termination signal strength at the fra meshift site, whereas the nucleotide in the +5 position had little inf luence, The efficiency of particular termination signals as a function of the +4 or +6 nucleotides correlates with how often they occur at n atural termination sites in E.coli; strong signals occur more frequent ly and weak signals are less common.