Ccm. Vanoers et al., THE EXON-4 POLY(A) SITE OF THE HUMAN CALCITONIN CGRP-I PRE-MESSENGER-RNA IS A WEAK SITE IN-VITRO, Biochimica et biophysica acta, N. Gene structure and expression, 1218(1), 1994, pp. 55-63
The human calcitonin/CGRP-I (CALC-I) pre-mRNA is processed in a tissue
-specific alternative way into either calcitonin (CT) or calcitonin ge
ne-related peptide-I (CGRP,I) mRNA. The exons 1 to 3 are common exons.
They are spliced to exon 4, which becomes polyadenylated to form CT m
RNA, or to exon 5 and the polyadenylated exon 6 to form CGRP-I mRNA. P
olyadenylation at exon 4 and splicing of exon 3 to exon 5 are mutually
exclusive processing reactions. Only splicing of exon 3 to exon 5 was
detected in vitro, with a minigene containing the exon 3 to exon 5 re
gion. No polyadenylation at the exon 4 poly(A) site could be observed.
Investigation of the properties of the exon 4 poly(A) site in vitro s
hows that it is inefficiently used in vitro. Cleavage and polyadenylat
ion of short RNAs containing only the exon 4 poly(A) site is strongly
dependent on the 3' length of the RNA. Downstream sequences located wi
thin 39 nucleotides from the cleavage site are required for optimal cl
eavage and polyadenylation. When the exon 4 poly(A) site in the minige
ne is replaced with the strong adenovirus L3 or rabbit beta-globin pol
y(A) sites, these sites can be efficiently used in vitro.