Translation in Eukaryotes
The process of translation in eukaryotes is essentially the same as that in
bacteria, but differs in several important ways. Structurally, the ribosomal
subunits of eukaryotes consist of 40S and 60S subunits that together
form an 80S complex. Although most bacterial mRNAs specify multiple
proteins, eukaryotic mRNAs code for a single nascent polypeptide chain.
However, some newly synthesized polypetide chains may subsequently
be enzymatically cleaved into two or more functional protein components.
Only three well-defined initiation factors are required for translation
of E. coli mRNAs, but many more are needed in eukaryotes. During initiation,
a special initiator tRNA (tRNAiMet) brings an unformylated methionine
into the first position on the ribosome. In eukaryotes the 40S ribosomal
subunit is thought to attach at the capped 5' terminus rather than
at a Shine-Dalgarno sequence as in prokaryotes. It then slides along until
it reaches the first AUG start codon. Three different elongation factors
in eukaryotes replace those found in bacteria. However, a single release
factors acts in eukaryotes.