Regulation of Gene Expression by Site-Specific Promoter Methylation

Abstract

It was established that the DNA encapsidated into purified adenovirions did not contain detectable levels of modified bases (Gunthert et al., 1976; Sutter et al., 1978; Eick et al., 1983). Even the DNA of a symmetric recombinant (Deuring et al., 1981; Deuring & Doerfler, 1983) between the left terminus of adenovirus type 12 DNA and large amounts of human cell DNA, which was encapsidated into virus particles, was not methylated (Deuring et al., 1981). The very same cellular DNA sequences as part of the human chromosomal DNA were heavily methylated (Deuring et al., 1981). Similarly, free viral DNA in human cells productively infected by human adenovirus type 2 or in hamster cells abortively infected by adenovirus type 12 was apparently not methylated at 5′-CCGG-3′ (HpaII), 5′-GCGC-3′ (HhaI), 5′-GATC-3′ (MboI), or 5′-TCGA-3′ (TaqI) sequences (Vardimon et al., 1980; Wienhues & Doerfler, 1985). These latter results were based on an extensive hybridization analysis in which the free intranuclear DNA was cleaved with the appropriate restriction endonucleases. Obviously, this type of study, though widely used, is limited due to the fact that, e.g. in the 35,937 base pairs of adenovirus type 2 DNA, only 22.5 % of the potentially methylated 5′-CG-3′ dinucleotide combinations reside in 5′-CCGG-3′ or 5′-GCGC-3′ sequences (Wienhues & Doerfler, 1985). Nevertheless, it appears highly unlikely that free intracellular adenovirus DNA in infected human or hamster cells would be extensively methylated. We have initiated a study of the actual patterns of methylation particularly of the major late promoter sequence of free adenovirus DNA at early and late times after the infection of cells by using the genomic sequencing technique (Church & Gilbert, 1984).