Identification of MRF4: a new member of the muscle regulatory factor gene family.

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Abstract
We have identified a rat cDNA encoding MRF4, a new member of the muscle regulatory factor gene family that includes MyoD1, myogenin, and Myf-5. MRF4 encodes a predicted 27-kD protein that contains a conserved helix-loop-helix motif, which is a common feature of this gene family. Northern analyses indicate that MRF4 is expressed solely in skeletal muscle tissue but is not detected in most embryonic muscle cell lines. Transfection of MRF4 into C3H10T1/2 fibroblasts produces stable myogenic lineages at frequencies that are equal to or greater than those obtained when MyoD1 or myogenin are introduced into these cells. Expression of the MRF4 cDNA leads to expression of the endogenous MyoD1 and myogenin genes, although C3H10T1/2 cells expressing MyoD1 or myogenin cDNAs do not express MRF4. Interestingly, the endogenous MyoD1 and myogenin genes are negatively regulated by serum and by purified growth factors since MRF4-transfected C3H10T1/2 cells activate MyoD1 and myogenin expression only in mitogen-depleted, differentiation-induced muscle cultures. The myofiber-specific expression pattern of MyoD1 and myogenin in these cells suggests that the primary role for this muscle regulatory factor gene family may be in regulating specific terminal differentiation events that are crucial for normal skeletal muscle development.