Regulation of DNA binding activity and nuclear transport of B-Myb in Xenopus oocytes

DNA binding activity and nuclear transport of B-Myb in Xenopus oocytes are negatively regulated. Two distinct sequence elements in the C-terminal port ion of the protein are responsible for these different inhibitory activitie s. A C-terminal Xenopus B-Myb protein fragment inhibits the DNA binding act ivity of the N-terminal repeats in trans, indicating that intramolecular fo lding may result in masking of the DNA binding function. Xenopus B-Myb cont ains two separate nuclear localization signals (NLSs), which, in Xenopus oo cytes, function only outside the context of the full-length protein. Fusion of an additional NLS to the full-length protein overcomes the inhibition o f nuclear import, suggesting that masking of the NLS function rather than c ytoplasmic anchoring is responsible for the negative regulation of Xenopus B-Myb nuclear transfer. During Xenopus embryogenesis, when inhibition of nu clear import is relieved, Xenopus B-myb is preferentially expressed in the developing nervous system and neural crest cells. Within the developing neu ral tube, Xenopus B-myb gene transcription occurs preferentially in prolife rating, non-differentiated cells.