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Abstract
We report the cloning of a cDNA (xnf7) coding for a maternally expressed Xenopus protein
that becomes highly enriched in nuclei of the central nervous system during later
development and in nuclei of adult brain. The protein also shows stage-specific nuclear/cytoplasmic
partitioning and phosphorylation that may be related to its function. In addition,
it binds to double-stranded DNA in vitro. The conceptual protein produced by the xnf7
clone contains several acidic domains, a novel zinc finger domain, three putative
p34cdc2 protein kinase phosphorylation sites, and a bipartite basic nuclear localization
signal. The xnf7 mRNA was detected as a maternal transcript that decreased in abundance
during development through the gastrula stage. It was reexpressed at the neural stage
in mesoderm and neural tissues, and its reexpression was not dependent upon the normal
juxtaposition of the mesoderm and ectoderm that occurs during neural induction as
demonstrated by high titer in exogastrulae. In situ hybridization showed enrichment
of the mRNA in the neural tube and a small amount in the mesoderm at the late neurula
stage. Xnf7 is normally phosphorylated during oocyte maturation. The bacterially expressed
xnf7 protein was phosphorylated in vitro by purified maturation-promoting factor at
a threonine in a small N-terminal domain containing one of the p34cdc2 protein kinase
phosphorylation sites, but not by several other protein kinases. The structural domains
present in the protein and its localization in nuclei suggest that the xnf7 gene product
performs an important nuclear function during early development, perhaps as a transcription
factor or a structural component of chromatin.