DNAase I sensitivity and methylation of active versus inactive rRNA genes in xenopus species hybrids.

We studied the chromatin structure and methylation of ribosomal RNA genes (rDNA) in hybrids between Xenopus laevis and Xenopus borealis. S1-nuclease protection experiments showed that 97%-98% of the rRNA precursor in hybrid tadpoles was of the X. laevis type. Preferential expression of the laevis rDNA was correlated with its hypersensitivity to DNAase I compared to borealis rDNA. Borealis and laevis rDNAs gave equivalent methylation patterns, however. The results show that hypomethylated sites in the nontranscribed spacer are not sufficient to ensure DNAase I hypersensitivity or transcription of the borealis rDNA. Also, heavy methylation of the transcribed region of laevis rDNA is compatible with its hypersensitivity to DNAase I. The absence of coupling between hypomethylation and DNAase I sensitivity argues against the view that the methylation pattern directly triggers the active chromatin structure, though it does not exclude a less intimate relationship between transcription and DNA hypomethylation. Examination of borealis sperm rDNA showed that hypomethylated sites were present at the same spacer locations as in somatic cells. This contrasts with X. laevis, where hypomethylated sites are detectable in the spacer of somatic rDNA, but not in sperm. Thus the loss of spacer methylation that is seen in early development of X. laevis does not occur in X. borealis.