SEQUENCE DIVERGENCE ASSOCIATED WITH SPECIES-SPECIFIC SPLICING OF THE NONMUSCLE BETA-TROPOMYOSIN ALTERNATIVE EXON

Alternative splicing of vertebrate beta-tropomyosin transcripts ensure s mutually exclusive expression of internal exons 6A and 6B in nonmusc le and skeletal muscle cells, respectively. Recently, we reported that this splicing regulation requires species-specific elements, since th e splicing profile for the chicken, rat, and Xenopus beta-tropomyosin alternative exons is not reproduced in transfection experiments when h eterologous myogenic cells are used. By analyzing the splicing pattern of hybrid chicken/rat beta-TM constructions transfected into both qua il and mouse cell lines, we demonstrate that chicken beta-tropomyosin exon 6A is flanked by stronger splicing signals than rat exon 6A, thus leading to the misregulation of splicing in heterologous cells. We ha ve characterized three splicing signals that contribute to this differ ence: 1) nonconsensus nucleotide differences at positions +4 and +6 in the donor site downstream of exon 6A, 2) differences in the pyrimidin e composition between the branch site and acceptor site upstream of ex on 6A, and 3) a pyrimidine-rich intronic exon 6A splicing enhancer pre sent upstream of exon 6A only in the chicken beta-TM gene. The functio nal divergence between splicing signals in two homologous vertebrate g enes reveals species-specific strategies for proper modulation of spli cing of alternative exons.