Differences between protein synthesis in eukaryotes and prokaryotes

In eukaryotes, the initiating amino acid is methionine (Met) and not fMet. The initiating tRNA, which responds only to AUG, is designated tRNAMetinit, or tRNAMetf to distinguish it from the tRNAMet, used in translating internal AUG codons.

At least nine initiation factors plus GTP are required for binding of tRNAMet,f, to the preinitiation complex. Two of these are similar to the factors in E. coli that prevent binding of the large subunit to the small subunit. One of these factors, eIF3, is a larger complex containing nine protein subunits and having a molecular weight of 7 X 105. An enzyme called initiating tRNA hydrolase, which cleaves the bond between methionine and tRNAMet,f removes the initial tRNA molecule after the first peptide bond forms, is also present in the complex. In prokaryotes, the corresponding enzyme, tRNA deacylase, is a ribosomal component.

Binding of tRNAMet,f, must occur before mRNA can bind, whereas for prokaryotes the mRNA can bind either before or after binding of the initiator tRNA. For binding of mRNA two other initiation factors are needed and ATP must be cleaved to form ADP and Pi. The reason for the cleavage of ATP is unknown. Binding occurs initially at or near the 5’ cap and is mediated by a cap binding factor (which is unnecessary for uncapped viral mRNA). The fact that the AUG codon nearest the 5’ terminus is almost always the initiating codon is a significant difference between prokaryotes and eukaryotes and plays important role in metabolic regulation.

More factors are needed for binding of the 60S subunit than for binding of the bacterial 50S subunit.

At least four elongation factors are needed by eukaryotes. These factors probably differ in structure and size in different tissues, often forming aggregates containing as many as 50 monomers.

Little is know about termination in eukaryotes, though release factors have been purified. Surprisingly, release in in vitro systems requires the presence of one of four tetranucleotides – UAAA, UAGA, UGAA or UAGG.

The most striking difference is that transcription and translation is not couple in eukaryotes whereas it is coupled in prokaryotes. In eukaryotes, the mRNA is synthesized in nucleus and then transported to cytoplasm where the ribosome located.

In prokaryotes, degradation of mRNA occurs continuously and while translation is in process. The half-life of a typical bacterial mRNA is about 1.8 minutes. Eukaryotic mRNA is very stable- possibly because of the 5’ cap. Degradation occurs very slowly and a typical half-life is several hours.

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