Transcription in oocytes of highly methylated rDNA from Xenopus laevis sperm

The genes for ribosomal RNA exist as multiple copies in the genome. Each repeated unit comprises a region that codes for the 40S rRNA precursor, and a spacer region of uncertain function (Fig. 1a). In Xenopus laevis there are about 1,000 copies of the dinucleotide sequence C-G in each repeat unit, and of these about 250 can be tested for the presence of 5-methylcytosine using restriction endonucleases. Most of the detectable C-Gs are heavily methylated, but in somatic cells unmethylated C-Gs occur in a 60 base pair (bp) sequence (NTS-60) that is repeated in the spacer. In contrast, the spacer of sperm rDNA is heavily methylated at these and all other testable C-Gs. Loss of methylation at NTS-60 takes place during the first day of embryonic development, near the time when rDNA transcription begins. In an attempt to assess the significance of this developmental change in methylation, we have isolated sperm rDNA and investigated whether it can be transcribed in oocytes. We have found that sperm rDNA is transcribed as efficiently as cloned rDNA, although no loss of methylation was detectable. Direct sequencing of sperm rDNA showed that all 19 C-GS in the promoter are highly methylated. Thus, in the case of rDNA injected into oocytes, loss of methylation is unnecessary for effective transcription.