DNA METHYLATION IN THE PROMOTER OF RIBOSOMAL-RNA GENES IN HUMAN-CELLSAS DETERMINED BY GENOMIC SEQUENCING

Many RNA polymerase II- or III-transcribed genes are inactive when the ir promoter is methylated at critical CpG dinucleotides. We have appli ed the genomic sequencing method and a direct DNA blotting technique t o analyze the extent of DNA methylation in the 5'-CpG-3' rich promoter region of the RNA polymerase I-transcribed ribosomal RNA genes (rDNA) in Dir;A from primary human cells, primary human tumor cells and huma n cell lines. In none of the analyzed primary human cells and primary human tumor cells was the DNA in the rDNA promoter region found to be detectably methylated. In contrast, in some of the cell lines this pro moter is methylated in all 5'-CpG-3' dinucleotides in the majority of the approximately 200 ribosomal RNA gene copies, In actively growing c ells, rDNA gene activity is a prerequisite for cell viability. The hig h levels of DNA methylation in the promoter region of rDNA in the huma n cell lines raise questions on the role of promoter methylation in th ese RNA polymerase I-transcribed genes, It is, however, conceivable th at a subset of the about 200 rDNA copies per haploid genome have escap ed methylation and account for the rRNA synthesis in these cell lines. Alternatively, complete 5'-CpG-3' promoter methylation may be compati ble with promoter activity as demonstrated for certain viral genomes.