9.5Cooperation between Transcription and Translation

Protein synthesis in prokaryotes begins when mRNA synthesis is still in progress. In genomic DNA, synthesis of the next mRNA molecule starts before the previous mRNA molecule is completely synthesized (Fig. 9-8). Protein synthesis is also initiated using the incompletely synthesized mRNA molecule. Occasionally, mRNA degradation proceeds from the 5′ terminus, while mRNA synthesis is still in progress at the 3′ terminus. Thus, the half-life of mRNA in E. coli is generally short, lasting only a few minutes.

Fig. 9-8 Prokaryotic translation initiated during transcription

In contrast, in eukaryotes, the synthesized pre-mRNA undergoes processing, and the completed mRNA is transported into the cytoplasm from the nucleus. Once in the cytoplasm, it is not necessarily immediately used in protein synthesis (see Selection 3 of Chapter 10, Fig. 10-3). This spatial and temporal separation of transcription and translation is a major difference between eukaryotes and prokaryotes. In eukaryotes, the half-life of mRNA ranges from dozens of minutes to very long periods.


Protein Synthesis Inhibitors

Many compounds inhibit protein synthesis. Protein synthesis inhibitors that act in prokaryotes are used as antibiotics. Some of these inhibitors, including chloramphenicol (chloromycetin), streptomycin, and tetracycline, are used as medicine (Column Fig. 9-1). The region of chloramphenicol enclosed in a dashed line is similar to a peptide bond; therefore, this antibiotic competitively inhibits the peptidyl transfer reaction. Streptomycin interferes with pairing between aminoacyl-tRNA and the mRNA codon. Tetracycline interferes with the binding of aminoacyl-tRNA to the ribosome. Puromycin acts on both prokaryotes and eukaryotes. The region enclosed in a dashed line is structurally similar to the 3′ terminus of aminoacyl-tRNA. Therefore, it enters the A site of a ribosome and is transferred to the growing peptide chain, causing release of the protein being synthesized. This inhibitor was often used to elucidate the mechanism of protein synthesis. Cycloheximide is an inhibitor of elongation reactions and acts specifically in eukaryotes. Diphtheria toxin is a protein toxin with enzyme activity that catalyzes ADP ribosylation of the histidine residue of eEF2 in the protein synthesis system, inactivating eEF2 to inhibit protein synthesis.

Column Fig. 9-1 Protein synthesis inhibitors

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