In prokaryotic cells, which have no nuclei, translation of an mRNA into protein can begin from the 5' end of the mRNA even while the 3' end is still being synthesized by RNA poly-merase. In other words, transcription and translation can occur concurrently in prokaryotes. In eukaryotic cells, however, not only is the nucleus separated from the cytoplasm where translation occurs, but also the primary transcripts of protein-coding genes are precursor mRNAs (pre-mRNAs) that must undergo several modifications, collectively termed RNA processing, to yield a functional mRNA (see Figure 4-1, step 2). This mRNA then must be exported to the cytoplasm before it can be translated into protein. Thus transcription and translation cannot occur concurrently in eukaryotic cells.
All eukaryotic pre-mRNAs initially are modified at the two ends, and these modifications are retained in mRNAs. As the 5' end of a nascent RNA chain emerges from the surface of RNA polymerase II, it is immediately acted on by several enzymes that together synthesize the 5' cap, a 7-methylguanylate that is connected to the terminal nucleotide of the RNA by an unusual 5',5' triphosphate linkage
(Figure 4-13). [-] cap protects an mRNA from enzymatic degradation and assists in its export to the cytoplasm. The cap also is bound by a protein factor required to begin translation in the cytoplasm.
Processing at the 3' end of a pre-mRNA involves cleavage by an endonuclease to yield a free 3'-hydroxyl group to which a string of adenylic acid residues is added one at a time by an enzyme called poly(A) polymerase. The resulting poly(A) tail contains 100-250 bases, being shorter in yeasts and invertebrates than in vertebrates. Poly(A) polymerase is part of a complex of proteins that can locate and cleave a transcript at a specific site and then add the correct number of A residues, in a process that does not require a template.
The final step in the processing of many different eu-karyotic mRNA molecules is RNA splicing: the internal cleavage of a transcript to excise the introns, followed by lig-ation of the coding exons. Figure 4-14 summarizes the basic steps in eukaryotic mRNA processing, using the p-globin gene as an example. We examine the cellular machinery for carrying out processing of mRNA, as well as tRNA and rRNA, in Chapter 12.
The functional eukaryotic mRNAs produced by RNA processing retain noncoding regions, referred to as 5' and 3'
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