FIG. 6.

Models for imprinting of target genes. The gametic methylation represents a simplified version of the average methylation seen at these stages. Black and white lollipops represent methylated and unmethylated CpG dinucleotides, respectively. (A) The remodeling of the maternal allele to a closed chromatin conformation may involve methyl binding proteins (MBPs) that recognize a target of densely methylated CpG sites, which then recruit histone deacetylases (HDAC). Since the closed chromatin conformation is maintained by a methylation-independent mechanism, the methylation state seen in the adult tissues can be variable. ORF, open reading frame; mat, maternal; pat, paternal. (B) The hypermethylation on the maternal allele blocks the binding of a transiently expressed transcriptional activator (TA) which binds only to the paternal allele. The TA keeps the paternal allele in a transcriptionally permissive state and allows transcription factors (TFs) to bind. In the absence of this TA, the maternal allele is remodeled to a closed chromatin state. (C) The positions of methylated CpG sites 13 to 22 found in the early embryo samples may become closely situated on the surface of a nucleosome and act as a target for MBPs.