During early amphibian embryogenesis, transcription from zygotic genes is repressed until the midblastula transition. DNA methylation is thought to underlie this repression: methyl-CpG-specific binding proteins interact with methylated DNA and recruit enzymes that promote the formation of inactive chromatin. Ruzov and colleagues now report that xKaiso, which binds specifically to methyl-CpG through a zinc-finger domain, maintains transcriptional silencing during the first 12 cleavage stages in Xenopus embryos (see p. 6185). The depletion of xKaiso function in early embryos by morpholinos results in zygotic gene expression beginning at least two cell cycles before the midblastula transition, and also in developmental arrest and apoptosis. This phenotype,which resembles that seen in hypomethylated embryos, is rescued by the injection of human kaiso mRNA. The researchers conclude that xKaiso controls this early genome-wide methylation-dependent transcriptional silencing, which is essential for amphibian development.

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