A BAC transgenic analysis of the Mrf4/Myf5 locus reveals interdigitated elements that control activation and maintenance of gene expression during muscle development

The muscle-specific transcription factors Myf5 and Mrf4 are two of the four myogenic regulatory factors involved in the transcriptional cascade respon sible for skeletal myogenesis in the vertebrate embryo. Myf5 is the first o f these four genes to be expressed in the mouse, We have previously describ ed discrete enhancers that drive Myf5 expression in epaxial and hypaxial so mites, branchial arches and central nervous system, and argued that additio nal elements are required for proper expression (Summerbell, D., Ashby, P. R., Coutelle, O., Cox, D., Yee, S, P, and Rigby, P, W J, (2000) Development 127, 3745-3757). We have now investigated the transcriptional regulation o f both MyfS and Mrf4 using bacterial artificial chromosome transgenesis, We show that a clone containing MyfS and 140 kb of upstream sequences is suff icient to recapitulate the known expression patterns of both genes. Our res ults confirm and reinforce the conclusion of our earlier studies, that MyfS expression is regulated differently in each of a considerable number of po pulations of muscle progenitors, and they begin to illuminate the evolution ary origins of this complex regulation, We further show that separate eleme nts are involved in the activation and maintenance of expression in the var ious precursor populations, reflecting the diversity of the signals that co ntrol myogenesis, Mrf4 expression requires at least four elements, one of w hich may be shared with Myf5, providing a possible explanation for the link age of these genes throughout vertebrate phylogeny, Further complexity is r evealed by the demonstration that elements which control Mrf4 and Myf5 are embedded in an unrelated neighbouring gene.