The rat myosin light chain (MLC)1/3 gene locus contains a potent muscle-specific enhancer, located downstream of the coding region, greater than 24 kilobases away from the MLC1 transcription start site. To assess the role of this enhancer in the activation of MLC expression during development, transgenic mice were generated carrying multiple copies of a MLC1 promoter-chloramphenicol acetyltransferase (CAT) transcription unit linked to a genomic fragment including the enhancer. CAT expression was detected in four mouse lines, up to 1000-fold higher in skeletal muscles than in other tissues. Activation of endogenous MLC1 transcription in these animals 4 days before birth was reflected in the onset of CAT transgene expression. This study identifies the transcriptional control elements necessary to activate the 21-kilobase MLC1/3 locus at the appropriate fetal stage and indicates that the MLC enhancer is sufficient to induce developmentally regulated expression from the MLC1 promoter exclusively in skeletal muscle cells.

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