Expression and regulation of chicken actin genes introduced into mouse myogenic and nonmyogenic cells.

We have introduced the chicken genes for cytoplasmic beta-actin, cardiac alpha-actin, and skeletal alpha-actin into C2 cells, a murine myogenic cell line, and into L cells by using the simian virus 40-derived vector PSV2 -gpt. In each selection, the entire population of transformed cells was analyzed for the expression and regulation of the actin genes by nuclease S1 assay and primer extension. This was compared to the expression of the vector marker Eco-gpt. The beta-actin gene is transcribed accurately and efficiently both in L-cells and in undifferentiated C2 cells. In fused C2 cells, beta-actin transcripts decrease significantly in parallel with the endogenous level of mouse beta-actin mRNA. Eco-gpt RNA levels remain essentially constant during myogenesis. The alpha-actin genes are correctly expressed at low levels in L cells but at significantly higher levels in the C2 cell background. Unlike the endogenous mouse alpha-actin gene, this level of expression does not change measurably with myogenesis. The skeletal alpha-actin gene is expressed poorly in pre- and post-fusion C2 cells, displaying no induction with differentiation. These results suggest that the tissue specificity of expression is maintained but the pattern of gene regulation for the sarcomeric actins is not. Factors in addition to the sequences flanking these genes are important for modulating gene expression during development. The decrease in the levels of beta-actin RNA during C2 cell differentiation provides a model system in which to study gene repression during development.