CONTROL OF CALCITONIN CALCITONIN-GENE-RELATED PEPTIDE PREMESSENGER RNA PROCESSING BY CONSTITUTIVE INTRON AND EXON ELEMENTS

The calcitonin/calcitonin gene-related peptide (CGRP) primary transcri pt is alternatively spliced in thyroid C cells and neurons, resulting in the tissue-specific production of calcitonin and CGRP mRNAs. Analys es of mutated calcitonin/CGRP transcription units in permanently trans fected cell lines have indicated that alternative splicing is regulate d by a differential capacity to utilize the calcitonin-specific splice acceptor. The analysis of an extensive series of mutations suggests t hat tissue-specific regulation of calcitonin mRNA production does not depend on the presence of a single, unique cis-active element but inst ead appears to be a consequence of suboptimal constitutive splicing si gnals. While only those mutations that altered constitutive splicing s ignals affected splice choices, the action of multiple regulatory sequ ences cannot be formally excluded. Further, we have identified a 13-nu cleotide purine-rich element from a constitutive exon that, when place d in exon 4, entirely switches splice site usage in CGRP-producing cel ls. These data suggest that specific exon recruitment sequences, in co mbination with other constitutive elements, serve an important functio n in exon recognition. These results are consistent with the hypothesi s that tissue-specific alternative splicing of the calcitonin/CGRP pri mary transcript is mediated by cell-specific differences in components of the constitutive splicing machinery.