Fig. 6.

Model for CHX action. E, P, and A sites of the 60S and 40S subunits are cartooned as open boxes; tRNAs are black lines; amino acids are blue, green, and yellow boxes; and CHX is a triangle in the 60S E site. (A) For a single-elongation cycle, in the absence of CHX, the aminoacyl-tRNA enters the empty A site (i), followed by nascent peptide transfer to the A-site tRNA (ii). Translocation occurs (iii and iv), and the deacylated tRNA leaves the E site to complete the cycle (v). (B) When CHX is bound in the E site, the aminoacyl-tRNA enters the empty A site (i), and transfer of the nascent peptide to the A-site tRNA occurs (ii). However, CHX in the E site prevents translocation of the P-site tRNA to the E site, arresting the ribosome in the PRE translocation state with the polypeptide attached to the A-site tRNA and deacylated tRNA in the P site. (C) For termination, a stop codon in the A site recruits release factor (i) resulting in release of the nascent peptide without translocation (ii). (D) When CHX is bound in the E site, termination occurs because translocation is not required.