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cDNA clones encoding four rat tropomyosin isoforms, termed TM-2, TM-3, TM-5a, and
TM-5b, were isolated and characterized. All are derived from the alpha-tropomyosin
gene via alternative RNA processing and the use of two alternate promoters. The cDNA
sequences predict that TM-2 and TM-3 both contain 284 amino acids and differ from
each other only at an internal region of the protein from amino acids 189 through
213, due to alternative splicing of exons 6a and 6b. TM-5a and TM-5b both contain
248 amino acids and differ from each other only at an internal exon encoding amino
acids 153 through 177, also due to alternative splicing of exons 6a and 6b. The differences
in the amino acid sequence encoded by these alternate exons affects the theoretical
actin-binding pattern of the tropomyosins, such that TM-5b is expected to bind actin
with greater affinity than TM-5a. TM-2 and TM-3 are transcribed from the upstream
promoter, and TM-5a and TM-5b are transcribed from an internal promoter. In addition,
all four isoforms contain the identical COOH-terminal coding region. RNA protection
analyses revealed that the mRNA for each isoform is expressed in a number of different
tissues and cell types, although the expression of some isoforms is restricted to
particular cell types. Furthermore, the expression of mRNA encoding these isoforms
was found to be altered in a number of different virally transformed cell lines. The
changes in the expression of tropomyosin mRNAs in transformed cells reflect changes
in the relative use of the two promoters, as well as the relative use of alternatively
spliced exons 6a and 6b.
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