FUNCTIONAL-ASPECTS OF B-MYB IN EARLY XENOPUS DEVELOPMENT

The gene encoding Xenopus B-Myb (XB-Myb), a protein structurally relat ed to the nuclear protooncogene product c-Myb, is expressed in early X enopus embryogenesis. We report on developmental alterations in the nu cleocytoplasmic distribution and phosphorylation of XB-Myb in Xenopus oocytes and embryos, as well as on a negative regulatory role of the c arboxyl terminus in sequence specific DNA binding. In growing oocytes and early embryonic stages the protein is primarily located in the nuc leus; in the full-grown oocyte, however, it remains sequestered in the cytoplasmic compartment. Upon meiotic maturation of the oocyte, XB-My b becomes hyperphosphorylated. Oocyte/egg isolates of XB-Myb are inhib ited in their specific DNA binding activity; truncation of the carboxy l terminal region relieves this block in nucleic acid recognition. Fur thermore, we have used overexpression of XB-Myb in Xenopus embryos by means of mRNA injection as an assay for gene function in vivo. Overexp ression of full-length XB-Myb, not of the carboxyl terminal deletion m utant, results in an altered morphology of lateral plate mesoderm.