Validation of an in vitro RNA processing system for CT/CGRP precursor mRNA.

The pre-mRNA encoding calcitonin (CT) and calcitonin gene-related peptide (CGRP) is differentially processed in a tissue-specific fashion to include or exclude the calcitonin-specific exon 4. A minigene containing a viral first exon and exons 4, 5, and 6 from the human CT/CGRP gene was correctly processed in transfected HeLa or F9 teratocarcinoma cells to produce mRNA that included or excluded exon 4, respectively. This processing decision could be reproduced in vitro using nuclear extracts from these two cell lines and an RNA precursor from a similar minigene. Supplementation of extract from HeLa cells with extract from F9 cells resulted in the F9 splicing pattern in which exon 4 was excluded. This model system may be useful for the purification of splicing factors important in the regulation of this splice choice.