A highly conserved enhancer downstream of the human MLC1/3 locus is a target for multiple myogenic determination factors

A potent muscle-specific enhancer element, originally described in the rat myosin light chain (MLC) 1/3 locus located downstream of the coding region, is found in an analogous position in the human MLC1/3 gene. When linked to a CAT reporter gene and transfected into muscle or non-muscle cells, the human MLC enhancer directs high levels of muscle-specific gene expression from homologous or heterologous promoters, irrespective of position or orientation relative to the CAT transcription unit. A significant degree of sequence homology (over 85%) in the 3'-flanking regions of the two MLC genes is restricted to a 200 bp sequence which lies approximately 1.5 kb downstream of the polyadenylation site in both species. The human enhancer sequence includes binding sites for human myogenic determination factors containing a common basic helix-loop-helix motif, and it can be trans-activated to varying degrees in non-muscle cells by these factors. This study establishes the MLC enhancer as an evolutionarily conserved, integral component of the MLC1/3 locus which constitutes a novel target for the action of myogenic determination factors.