The methylation pattern of tRNA genes in Xenopus laevis

The methylation sensitive restriction enzymes Hha I and Hpa II were used to analyse the methylation pattern of a tRNA gene cluster in germ line and somatic DNA of Xenopus laevis. A single tDNA repeat containing 8 tRNA genes was studied and all copies were found to be fully modified in sperm DNA. In the DNA from erythroid cells, hepatocytes, kidney and brain, most tDNA repeats were found to be fully modified. However, in a fraction of the repeats, specific demethylated sites can be detected, giving rise to a pattern which does not vary significantly from one tissue to another. Although our results do not allow a straightforward correlation between hypomethylation and tRNA gene transcription, they are in agreement with the observation that hypomethylation accompanies differentiation and development.