THE XENOPUS-LAEVIS TM-4 GENE ENCODES NONMUSCLE AND CARDIAC TROPOMYOSIN ISOFORMS THROUGH ALTERNATIVE SPLICING

A full-length cDNA clone coding for a 248-amino-acid tropomyosin (TM) was isolated from a Xenopus laevis (Xl) oocyte cDNA library. This TM i s very similar to the members of the non-muscle (nm) TM family that in cludes rat TM-4, human TM30pl and chicken cardiac fibroblast FT-C. An RNase protection assay showed that the corresponding transcript is exp ressed ubiquitously and revealed the presence of an alternative transc ript in adult heart RNA. A full-length cDNA clone was isolated from an adult heart cDNA library using a nm TM-encoding cDNA probe. It encode s a muscle TM homologous to chicken FT-C and the corresponding mRNA is expressed in adult RNA and to a very low level in adult skeletal musc le, The two cDNAs correspond to two alternatively spliced isoforms of TM generated from the Xl TM-4 gene. These data, together with publishe d observations, demonstrated that, in this amphibian, the cardiac musc le TM are synthesized from the TM alpha and TM-4 genes.