The segmentation of eukaryotic genes into exons and introns


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The segmentation of eukaryotic genes into exons and introns presents an opportunity for the production of multiple gene products from a single gene by alternative RNA processing. Developmental programs often use differential splicing or differential polyadenylation to produce tissue-specific variants from one transcription unit. The gene encoding the small peptide hormone calcitonin is one such differentially utilized gene. The calcitonin gene contains six exons. In thyroid cells an mRNA that encodes calcitonin is produced; it contains exons 1, 2, 3, and 4 and uses a polyadenylation site at the end of exon 4. In neuronal cells no calcitonin is produced from this gene. In neuronal cells calcitonin gene-related peptide (CGRP) is produced from this gene; its mRNA consists of exons 1,2,3,5, and 6. The gene and its tissue-specific pattern of processing are diagrammed in Figure 1. In both cell types transcription begins in the same place and extends beyond exon 6. The mechanism of differential processing of the calcitonin/CGRP transcript is not understood. Because different poly-A sites and different splice sites are utilized in the two processing pathways, the tissue-specific factors that regulate calcitonin and CGRP expression could be involved either in poly-adenylation or in splicing. There are two popular models. One is that thyroid cells produce calcitonin because they contain a specific factor that recognized the poly-A site in exon 4 with high efficiency and causes cleavage of the precursor RNA before splicing of exon 3 to exon 5 can occur. Neuronal cells lack this factor with the result that splicing of exon 3 to exon 5 predominates, leading to CGRP mRNA production. A second model is that splice-site selection determines which RNA is produced. Thyroid cells produce calcitonin because they splice exon 3 to exon 4; neuronal cells produce CGRP because they splice exon 3 to exon 5. Presumably, one or both cell types produce a factor that favors one splice over the other. To test these hypotheses, the splicing and polyadenylation signal at the ends of exon 4 were 3 altered by mutation (see Fig. 1). The altered genes were transfected into a lymphocyte cell line, which produces only calcitonin from the wild-type gene. The mutant lacking the exon-4 polyadenylation site produced no mRNA at all; the mutant lacking the exon-4 splice site produced only CGRP mRNA. A. Does the lymphocyte cell line contain the splicing and polyadenylation factors necessary to produce both calcitonin and CGRP mRNA? Why? B. If differential processing results from polyadenylation-site selection, which mutant would you expect to produce CGRP mRNA when transfected into the lymphocyte cell line? Why? C. If differential processing results from splice-site selection, which mutant would you expect to produce CGRP mRNA when transfected into the lymphocyte cell line? Why? D. Which model for differential processing best explains the ability of the lymphocyte cell line to produce calcitonin mRNA but not CGRP mRNA. Why?

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