Multiple Combinations of Alternatively Spliced Exons in Rat Tropomyosin-α Gene mRNA: Evidence for 20 New Isoforms in Adult Tissues and Cultured Cells

Previous studies of the tropomyosin-α gene using Northern blot and ribonuclease protection assay methods identified the expression of nine isoforms generated by alternative splicing of exons. Several of these isoforms were characterized as tissue-specific and/or developmentally specific. The present study used a highly sensitive RT-PCR-based strategy to assay the expression of these and many novel isoforms in a variety of adult rat tissues. All 9 isoforms were found to be expressed in all tissues evaluated. Furthermore, 20 new isoforms were identified with varying tissue specificity. Sequence analysis confirmed exon splicing patterns. This greater degree of isoform generation parallels recent findings for another tropomyosin gene, the TM-5 gene, for which the generation of new isoforms, in particular, ones using novel junctions for carboxy-terminal-coding exons, was also shown. Several of the new cDNA-based isoforms predict tropomyosin protein species that are 10 amino acids longer than previously characterized high-molecular-weight tropomyosin-α gene isoforms. The apparent lack of significant tissue specificity in the expression of tropomyosin isoforms suggests that many of these isoforms have more generic roles in cell function.