ABSTRACT
A monoclonal antibody specific for Xenopus MyoD (XMyoD) has been characterized and used to describe the pattern of expression of this myogenic factor in early frog development. The antibody recognizes an epitope close to the N terminus of the products of both XMyoD genes, but does not bind XMyfS or XMRF4, the other two myogenic factors that have been described in Xenopus. It reacts in embryo extracts only with XMyoD, which is extensively phosphorylated in the embryo. The distribution of XMyoD protein, seen in sections and whole-mounts, and by immunoblotting, closely follows that of XMyoD mRNA. XMyoD protein accumulates in nuclei of the future somitic mesoderm from the middle of gastrulation. In neurulae and tailbud embryos it is expressed specifically in the myotomal cells of the somites. XMyoD is in the nucleus of apparently every cell in the myotomes. It accumulates first in the anterior somitic mesoderm, and its concentration then declines in anterior somites from the tailbud stage onwards.
