SPLICING OF 2 INTERNAL AND 4 CARBOXYL-TERMINAL ALTERNATIVE EXONS IN NONMUSCLE TROPOMYOSIN-5 PRE-MESSENGER-RNA IS INDEPENDENTLY REGULATED DURING DEVELOPMENT

Four nonmuscle tropomyosin isoforms have been reported to be produced from the rat Tm5 gene by alternative splicing (Beisel, K, W., and Kenn edy, J. E, (1994) Gene (Amst,) 145, 251-256), In order to detect addit ional isoforms that might be expressed from that gene, we used reverse transcriptase-polymerase chain reaction assays and evaluated the pres ence of all product combinations of two alternative internal exons (6a and 6b) and four carboxyl-terminal exons (9a, 9b, 9c, and 9d) in deve loping and adult rat brain. We identified five different combinations for exon 9 (9a + 9b, 9a + 9c, 9a + 9d, 9c, and 9d), and the exon combi nations 9a + 9c and 9a + 9d were previously unreported. Each of these combinations existed with both exon 6a and exon 6b, Thus, the rat brai n generates at least 10 different isoforms from the Tm5 gene. Northern blot hybridization with alternative exon specific probes revealed tha t these isoforms were also expressed in a number of different adult ra t tissues, although some exons are preferentially expressed in particu lar tissues. Studies of regulation of the 10 different Tm5 isoform mRN As during rat brain development indicated that no two isoforms are coo rdinately accumulated. Furthermore, there is a developmental switch in the use of exon 6a to exon 6b from embryonic to adult isoforms, TM5 p rotein isoforms show a differential localization in the adult cerebell um.