A cDNA library was constructed from mouse cardiac muscle mRNA, and a clone corresponding to part of the mRNA for the cardiac muscle isoform of actin was isolated from this library. The nucleotide sequence of the cloned insert was determined and was found to contain almost the complete amino acid coding region for actin (only codons for the first two amino acids, absent from the mature protein, were lacking) and a substantial portion derived from the 3′ untranslated region of the mRNA. Comparison of the latter with the corresponding region in cardiac actin mRNA from man and rat showed that this 3′ untranslated region has been subject to conservational pressure during evolution. However a comparison with the corresponding region in skeletal muscle actin mRNAs indicated that the pattern of conservation is quite different in the two striated muscle actin isoforms.

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