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

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Abstract
The chicken cytoplasmic β actin gene is ubiquitously expressed in all cell types. In terminally differentiated muscle cells, however, the concentration of β actin specific mRNA is down-regulated to scarcely detectable levels. To test for gene regions which are involved in the muscle specific reduction of β actin specific mRNA, the isolated complete chicken β actin gene or chimeric gene constructs containing parts of the gene were stably trsnsfected into the myogenic mouse cell line C2C12 and their transcriptional activity was compared in proliferating myoblasts end postmitotic myotubes. A hybrid construct containing the β 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 β actin gene led to a marked reduction of β actin transcripts in differentiated C2C12 myotubes. The stability of β 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 β actin gene confers the rnyotube specific down-regulation that is not primarily due to destabili zation of mRNA.