MYOSIN LIGHT-CHAIN 3F REGULATORY SEQUENCES CONFER REGIONALIZED CARDIAC AND SKELETAL-MUSCLE EXPRESSION IN TRANSGENIC MICE

The myosin light chain 1F/3F locus contains two independent promoters, MLC1F and MLC3F, which are differentially activated during skeletal m uscle development. Transcription at this locus is regulated by a 3' sk eletal muscle enhancer element, which directs correct temporal and tis sue-specific expression from the MLC1F promoter in transgenic mice. To investigate the role of this enhancer in regulation of the MLC3F prom oter in vivo, we have analyzed reporter gene expression in transgenic mice containing lacZ under transcriptional control of the mouse MLC3F promoter and 3' enhancer element. Our results show that these regulato ry elements direct strong expression of lacZ in skeletal muscle; the t ransgene, however, is activated 4-5 d before the endogenous MLC3F prom oter, at the time of initiation of MLC1F transcription. In adult mice, transgene activity is downregulated in muscles that have reduced cont ributions of type IIB fibers (soleus and diaphragm). The rostrocaudal positional gradient of transgene expression documented for MLC1F trans genic mice (Donoghue, M., J. P. Merlie, N. Rosenthal, and J. R. Sanes. 1991. Proc. Natl. Acad. Sci. USA. 88:5847-5851) is not seen in MLC3F transgenic mice. Although MLC3F was previously thought to be restricte d to skeletal striated muscle, the MLC3F-lacZ transgene is expressed i n cardiac muscle from 7.5 d of development in a spatially restricted m anner in the atria and left ventricular compartments, suggesting that transcriptional differences exist between cardiomyocytes in left and r ight compartments of the heart. We show here that transgene-directed e xpression of the MLC3F promoter reflects low level expression of endog enous MLC3F transcripts in the mouse heart.