2 DIFFERENT SEQUENCE ELEMENTS WITHIN EXON-4 ARE NECESSARY FOR CALCITONIN-SPECIFIC SPLICING OF THE HUMAN CALCITONIN CALCITONIN-GENE-RELATED PEPTIDE-I PRE-MESSENGER-RNA

The calcitonin (CT)/calcitonin gene-related peptide I (CGRP-I) gene (C ALC-I gene) is subject to alternative tissue-specific processing of it s primary transcript. CT mRNA is the predominant mRNA produced in thyr oid C tells, whereas CT gene-related peptide I mRNA is the main produc t in neurons of the central and peripheral nervous systems. The CT-spe cific exon 4 is surrounded by weak processing sites. In this study we have investigated whether exon 4 sequences are involved in the tissue- specific selection of the exon I splice acceptor site. The results ind icate that two separate elements, termed A and B, in the 5' part of ex on 4 are required for production of CT-specific RNA. These sequences a re located between nucleotides 67 and 88 (A) and nucleotides 117 and 1 46 (B) relative to the 5' end of exon 4. Variation of the distance bet ween these sequence elements and the 3' splice site of exon 4 does not change the processing choice. These sequence elements are functionall y equivalent. CT-specific splicing requires the presence of both seque nce A and B or duplicates of either sequence element in exon 4. The ef fect of these sequences on the RNA processing choice is overruled by m utation of the CT-specific uridine branch acceptor nucleotide into a c ommonly preferred adenosine residue.