Inheritance of DNA methylation in microinjected eggs of Xenopus laevis.

Plasmid DNA methylated at Hpa II sites was injected into eggs of Xenopus laevis. The methylated DNA replicates extrachromosomally, and the methylated state of the H pa II sites is inherited through replication. Unmethylated sites remain unmethylated in progeny molecules. To test whether replication is necessary for new methylation to occur, DNA methylated on one strand only was injected and unreplicated DNA was selected for analysis. Methylation was copied onto a previously unmethylated strand in the absence of replication but less efficiently than in its presence. These experiments show that the individual methylated site contains enough information for inheritance of the methylated state and that inheritance of methylation does not require integration of the DNA into the chromosome.