The shape of the ribosome recycling factor resembles the shapes of the elongation and termination factors, and probably indicates that it binds the ribosome in a similar location. This process of releasing the ribosome after protein synthesis has terminated is fundamental because the gene for RRF is essential for cell growth . If frameshifting occurs, the complete sequence of RF-2 is synthesized. Comparison of amino acid and mRNA sequences showed that ribosomes can shift reading frame at codon 25, avoid the stop codon, and decode the main portion of the message. Competition between termination and frameshifting is over which process captures the U of the UGA sequence.
Competition is tilted in favor of recoding in two ways.
The presence of a C 3' of the UGA makes a poor termination context, and CUU is a particularly shift-prone codon. If extra RF2 is present, frameshifting decreases so that less RF-2 is made. When RF2 is in short supply termination loses, frameshifting wins, and RF-2 concentration is restored. The stimulatory signal is a short sequence three nucleotides 5' of the shift site that pairs with 16S rRNA of the translating ribosome, just like the Shine-Dalgarno pairing that occurs 5' of the AUG start codon during ribosome initiation. Spacing between the Shine-Dalgarno sequence and the site of action is crucial and varies for these cases.