Loss of rDNA methylation accompanies the onset of ribosomal gene activity in early development of X. laevis.

The rRNA genes of Xenopus blood cells are heavily methylated, but there are two regions in the spacer that frequently contain unmethylated CpG. The undermethylated regions coincide with two regions containing a 60 nucleotide tandemly repeated sequence, and they are present in all somatic tissues that we have tested. Sperm rDNA, by contrast, is fully methylated at these sites in the spacer, and indirect evidence suggests that this may also be the case in oocytes. Loss of methyl groups occurs progressively over the first 20 hr of development, the same period in which embryonic rRNA synthesis initiates and increases in rate.