Alternative splicing of Xenopus alpha(fast)-tropomyosin pre-mRNA during development: Identification of determining sequences

The Xenopus alpha(fast)-tropomyosin gene contains in its central part a set of mutually exclusive exons, designated 6A and 6B, which are incorporated into mRNA encoding, respectively, nonmuscle and muscle tropomyosins. In thi s study, we show that usage of both exons is strictly regulated during deve lopment, exon 6A being used in the oocyte and nonmuscle tissues of the embr yo, while exon 6B is used in muscle tissues. An approach of transient embry o transgenesis was developed to study the mechanisms involved in the splice site choice during development. We demonstrate that alpha-tropomyosin mini genes driven by tissue-specific promoters that target gene expression in no nmuscle and muscle tissues recapitulate the splicing pattern of the endogen ous gene. A mutational analysis showed that regulation occurred at both exo ns 6A and 6B in muscle and nonmuscle tissues. In this context, we have iden tified an element located in the intron downstream of 6A that participates in the recognition of the weak 5' splice site of exon 6A and the repression of exon 6B in nonmuscle cells.