(030717 revised, 030320 created)
Codon Capture Theory
A model for evolution of UGA Trp codon by codon capture in Mycoplasma capricolum.
(a) Ancestor of Mycoplasma capricolum. UGG was the sole Trp codon translated by
anticodon CCA. UGA (and UAA) was a stop codon recognized by RF (release
factor)-2. (b) RF-2 disappeared and anticodon CCA duplicated. (c) Through
AT-biased mutation pressure, one of the anticodons, CCA, mutated to UCA. (d) A
large fraction of the in-frame UGG Trp codons mutated to UGA, so that UGA
together with a small amount of UGG became translatable by newly produced
anticodon UCA by wobbling. Anticodon CCA became no more needed and is in the
course of disappearance by AT-pressure. Thus, UGA codon was completely removed
from the termination site, and was captured by Trp. The line (d) represents the
present state of Mycoplasma capricolum.. Now, stop codon UAA is
recognized by RF-1. (e) Anticodon CCA will disappear in the near future. The
process from (a) to (e) is neutral, because amino acid sequence remains
unchanged