Dial M(RF) for myogenesis.

The transcriptional regulatory network that controls the determination and differentiation of skeletal muscle cells in the embryo has at its core the four myogenic regulatory factors (MRFs) Myf5, MyoD, Mrf4 and MyoG. These basic helix-loop-helix transcription factors act by binding, as obligate heterodimers with the ubiquitously expressed E proteins, to the E-box sequence CANNTG. While all skeletal muscle cells have the same underlying function their progenitors arise at many sites in the embryo and it has become apparent that the upstream activators of the cascade differ in these various populations so that it can be switched on by a variety of inductive signals, some of which act by initiating transcription, some by maintaining it. The application of genome-wide approaches has provided important new information as to how the MRFs function to activate the terminal differentiation programme and some of these data provide significant mechanistic insights into questions which have exercised the field for many years. We also consider the emerging roles played by micro-RNAs in the regulation of both upstream activators and terminal differentiation genes.