Recombinant DNA clones containing chick α-actin mRNA sequence have been isolated and used as probes to analyze the structure and developmental expression of the chick α-actin gene. The full length, 2000 nucleotide α-actin mRNA is detected in poly(A) RNA at early and late stages of in vivo leg muscle development. As expected, the α-actin mRNA is present at very low levels at early myogenic stages but is a high abundance species in terminally differentiated muscle. However, most of the α-actin mRNA from fused leg muscle is shorter than 2000 nucleotides, and occurs in relatively discrete size classes. An α-actin-like mRNA can be detected in poly(A) RHA from early embryonic brain, indicating that transcription of the α-actin gene may not be strictly muscle-specific at all stages of development.We have identified at least 3, very short (< 100 base pairs) intervening sequences in the α-actin gene which was isolated from a chick genomic library. The structure of the chick α-actin gene differs, therefore, from the structures of actin genes from yeast and Drosophila, both of which contain a single, relatively long, intervening sequence.
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