Eukaryotic
vs Prokaryotic Translation
The mechanism for translating messenger RNA into protein in eukaryotic cells is basically the same as in prokaryotes. That is, messenger RNA (mRNA) is read by ribosomes. There are, however, significant differences in both the ribosomes and the details of the translational mechanism.
Ribosomes - The ribosome and its subunits are larger in eukaryotes. 40S and 60S subunits combine to form a functional 80S ribosome. In prokaryotes, the analogous particles are 30S, 50S, and 70S, respectively. The large eukaryotic ribosomal subunit (60S) contains 28S (26S in yeast), 5S, and 5.8S rRNAs, the last having no counterpart in prokaryotes. The small subunit (40S) has an 18S rRNA (17S in yeast) versus 16S rRNA in prokaryotes. The eukaryotic ribosomal subunits also contain more proteins than the corresponding prokaryotic particles do.
Initiation - Initiation requires many more protein factors (Table 28.7) in eukaryotes than in prokaryotes (11 versus 3)-compare Figure 28.35 with Figure 27.20, for example. Some of the initiation factors attach to the ribosomal subunits and others to mRNA (see left-hand side of Figure 28.35). The major initiation factor, eIF2, forms a complex with tRNA. This factor is eventually recycled via a cyclic GDP--GTP exchange called the eIF2 cycle, which requires the factor eIF2B. Eukaryotic protein synthesis is initiated as it is in prokaryotes, with a special Met-tRNA reading an AUG codon, but the methionine is not formylated. The mRNA is aligned correctly on the 40S ribosomal subunit by the 5' cap, rather than by the Shine--Dalgarno sequence (see here) used by prokaryotes. The ribosomal subunit then scans along the mRNA (an ATP-dependent process) until the first AUG is found. At this point the initiation factors are released, and the 60S subunit is attached to begin translation.
Elongation and termination - Eukaryotic chain termination, in contrast to prokaryotic termination, requires only one protein factor- eRF (Table 28.7), which can recognize all three stop codons (UAA, UAG, and UGA). Otherwise the mechanisms are very similar.
Inhibitors of translation - A number of the common inhibitors of prokaryotic translation are also effective in eukaryotic cells. They include pactamycin, tetracycline, and puromycin. Inhibitors that are effective only in eukaryotes include cycloheximide and diphtheria toxin. Cycloheximide inhibits the peptidyltransferase activity of the eukaryotic ribosome. Diphtheria toxin is an enzyme, coded for by a bacteriophage that is lysogenic in the bacterium Corynebacterium diphtheriae. It catalyzes a reaction in which NAD+ adds an ADP ribose group to a specially modified histidine in the translocation factor eEF2, the eukaryotic equivalent of EF-G (Figure 28.36). Because the toxin is a catalyst, minute amounts can irreversibly block a cell's protein synthetic machinery. As a result, pure diphtheria toxin is one of the most deadly substances known.
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