The down-regulation of the chicken cytoplasmic beta actin during myogenic differentiation does not require the gene promoter but involves the 3' end of the gene.

The chicken cytoplasmic beta actin gene is ubiquitously expressed in all cell types. In terminally differentiated muscle cells, however, the concentration of beta actin specific mRNA is down-regulated to scarcely detectable levels. To test for gene regions which are involved in the muscle specific reduction of beta actin specific mRNA, the isolated complete chicken beta actin gene or chimeric gene constructs containing parts of the gene were stably transfected into the myogenic mouse cell line C2C12 and their transcriptional activity was compared in proliferating myoblasts and postmitotic myotubes. A hybrid construct containing the beta actin promoter fused to the bacterial CAT gene showed high and constitutive expression during myocyte differentiation. In contrast, constructs containing the SV40 early promoter linked to the 3' end of the beta actin gene led to a marked reduction of beta actin transcripts in differentiated C2C12 myotubes. The stability of beta actin mRNA was analyzed in actinomycin D treated cells and found to be virtually unchanged in myotubes as compared to myoblasts. These results suggest that a sequence element located in the 3' end or 3' flanking region of the beta actin gene confers the myotube specific down-regulation that is not primarily due to destabilization of mRNA.