Regulation and functions of myogenic regulatory factors in lower vertebrates.

The transcription factors of the MyoD family have essential functions in myogenic lineage determination and muscle differentiation. These myogenic regulatory factors (MRFs) activate muscle-specific transcription through binding to a DNA consensus sequence known as the E-box present in the promoter of numerous muscle genes. Four members, MyoD, myogenin, myf5 and MRF4/herculin/myf6, have been identified in higher vertebrates and have been shown to exhibit distinct but overlapping functions. Homologues of these four MRFs have also been isolated in a variety of lower vertebrates, including amphibians and fish. Differences have been observed, however, in both the expression patterns of MRFs during muscle development and the function of individual MRFs between lower and higher vertebrates. These differences reflect the variety of body muscle formation patterns among vertebrates. Furthermore, as a result of an additional polyploidy that occurred during the evolution of some amphibians and fish, MyoD, myogenin, myf5 and MRF4 may exist in lower vertebrates in two distinct copies that have evolved separately, acquiring specific roles and resulting in increased complexity of the myogenic regulatory network. Evidence is now accumulating that many of the co-factors (E12, Id, MEF2 and CRP proteins) that regulate MRF activity in mammals are also present in lower vertebrates. The inductive signals controlling the initial expression of MRFs within the developing somite of lower vertebrate proteins are currently being elucidated.