Nucleases and RNA Turnover

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Figure 16.27 Alternate splicing of tropomyosin gene transcripts results in a family of tissue­specific tropomyosin proteins. Redrawn from Breitbart, R. E., Andreadis, A., and Nadal­Ginard, B. Annu. Rev. Biochem. 56:467, 1986.
Tropomyosin proteins are essential components of the contractile apparatus in the three types of muscle (see, p. 948) and each contractile cell type contains a specific tropomyosin type. This diversity arises from a single gene that is transcribed into a primary transcript. The transcript is then processed as diagramed in Figure 16.27. All cells containing tropomyosin make the same primary transcript but each cell type processes this transcript in a characteristic fashion. The resulting mRNA species then are translated to yield the tropomyosins characteristic of each cell type. About 40 examples are well documented of tissue­specific splicing. Thus the existence of introns supplies the organism with still another method of generating protein diversity.
16.6— Nucleases and RNA Turnover
The different roles of RNA and DNA in genetic expression are reflected in their metabolic fates. A cell's information store (DNA) must be preserved, thus the myriad DNA repair and editing systems in the nucleus. Although individual stretches of nucleotides in DNA may turn over, the molecule as a whole is metabolically inert when not replicating. The various RNA molecules, on the other hand, are individually dispensable and can be replaced by newly synthe­