Initiation Mechanisms Used in the Translation of Bicistronic mRNAs

Abstract

The mechanism and control of eukaryotic protein synthesis have been the topic of many review articles in the past few years.^1–15 While some have dealt in fine detail with parts of the process, others have focused more on control than mechanism. In large part, all of these reviews have dealt with what is considered “normal” eukaryotic protein synthesis. This begins with a messenger RNA (mRNA) with a 5′ terminal m⁷G cap structure, usually a 5′ untranslated region of 30–150 bases followed by a coding region which begins with the Met-transfer RNA: (tRNA_i) initiating AUG codon in the following favored context: A/GXXAUGG. The mRNA usually contains several hundred bases 3′ to the coding region and is followed by a poly(A) tail, where the poly(A) length is 100–150 bases. A generalized pathway describing the sequential utilization of translation factors has been proposed to account for the binding of eukaryotic mRNAs to ribosomes.1 In this pathway, eIF-2 directs the binding of Met-tRNA_i to the 40S subunit prior to the binding of mRNA. Binding of the mRNA to the 40S subunit is achieved by interaction with the three-subunit factor eIF-4F, which specifically recognizes the m⁷G cap structure. Subsequently, eIF-4A and eIF-4B, in the presence of ATP, serve to unwind the mRNA and allow for its attachment to the 40S subunit. A correct match of the initiator tRNA with the correct AUG start code word appears to be accomplished by the ATP-dependent process termed scanning,^7,15 whereby the 40S subunit moves in a 5′ to 3′ direction in search of the initiating AUG. Current reports^16–18 support the hypothesis that recognition of the initiating AUG codon is effected via the anticodon of the initiator tRNA with the AUG codon.