SP1 SITES IN THE MOUSE APRT GENE PROMOTER ARE REQUIRED TO PREVENT METHYLATION OF THE CPG ISLAND

In an attempt to find the mechanism by which CpG islands remain free o f methylation we have undertaken a detailed examination of the mouse a denine phosphoribosyltransferase (aprt) gene. This housekeeping gene h as a CpG island that extends over the gene promoter and includes the f irst two exons. We show that the island is free of methylation at all CpGs, whereas the flanks are methyated, Detailed patterns of methylati on beyond the boundaries of the CpG island vary between cells. In vivo footprinting across the island region shows that three GC boxes clust ered at the 5' edge of the CpG island are occupied, most probably by S pl. No other footprints are detected within the island region. Deletio n or mutagenesis of the Spl sites causes de novo methylation of the Cp G island in a transgenic mouse assay. Thus, the peripherally located S pl sites are necessary to keep the aprt island methylation free.