XENOPUS-EMBRYOS REGULATE THE NUCLEAR-LOCALIZATION OF XMYOD

Injection of Xenopus myoD mRNA into Xenopus embryos leads to only a mo dest activation of myogenic markers. In contrast, we show that injecte d mouse myol mRNA leads to a potent activation. We postulate that XMyo D is under negative control in frog embryos, but because of sight sequ ence differences, mouse MyoD fails to see the negative signal. Whereas mMyoD is constitutively nuclear, XMyoD is largely cytoplasmic except in a region of the embryo that includes the location where mesoderm in duction occurs; there, it is nuclear. At MBT, endogenous XmyoD mRNA is expressed ubiquitously in the frog embryo. Our results suggest that t his expression would lead to cytoplasmic XMyoD protein. Among other ev ents, muscle induction might remove this negative regulation, allow My oD to enter the nucleus, and establish an autoregulatory loop that cou ld commit cells to myogenesis.