EMBRYONIC AND FETAL MYOGENIC PROGRAMS ACT THROUGH SEPARATE ENHANCERS AT THE MLC1F 3F LOCUS/

Embryonic and fetal stages of skeletal muscle development are characte rized by the differential expression of a number of muscle-specific ge nes. These include the products of independent promoters at the fast m yosin light chain 1F/3F locus. In the mouse embryo MLC1F transcripts a ccumulate in embryonic skeletal muscle from E9, 4-5 days before high-l evel accumulation of MLC3F transcripts. A 3' enhancer can activate MLC 1F and MLC3F promoters in differentiated muscle cells in vitro and in transgenic mice; both promoters, however, are activated at the time of MLC1F transcript accumulation. We now demonstrate the presence of a s econd muscle-specific enhancer at this locus, located in the intron se parating the MLC1F and MLC3F promoters. Transgenic mice containing the intronic, but lacking the 3' enhancer, express high levels of an nlac Z reporter gene from the MLC3F promoter in adult fast skeletal muscle fibers. In contrast to the 3' enhancer, the intronic element is inacti ve both in embryonic muscle cells in vive and in embryonic myocyte cul tures. The intronic enhancer is activated at the onset of fetal develo pment in both primary and secondary muscle fibers, at the time of endo genous MLC3F transcript accumulation. Late-activated MLC3F transgenes thus provide a novel in tote marker of fetal myogenesis. These results suggest that temporal regulation of transcription at the MLC1F/3F loc us is controlled by separate enhancers which are differentially activa ted during embryonic and fetal development. (C) 1997 Academic Press.