NEGATIVE AND POSITIVE EFFECTS OF CPG-METHYLATION ON XENOPUS RIBOSOMALGENE-TRANSCRIPTION IN-VITRO

Methylation of cytosine-residues in the sequence CpG affects the expre ssion of many genes and generally correlates with reduced transcriptio n. The ribosomal genes of Xenopus laevis were among the first genes to be studied with respect to their DNA methylation, and a loss of methy lation during embryonic development correlated with the onset of trans cription. Nevertheless, highly methylated ribosomal genes were transcr ibed at normal levels when injected into oocyte nuclei, and thus trans cription of these genes was generally assumed to be insensitive to CpG -methylation. Here I show that Xenopus ribosomal gene transcription ca n be repressed by cellular factors binding to (me)CpG, similarly as it has been described for transcription by RNA polymerase II. In the abs ence of these repressors, however, CpG-methylation has a direct positi ve effect on RNA polymerase I-promoter activity.