The unmethylated state of the promoter/leader and 5'-regions of integrated adenovirus genes correlates with gene expression.

An inverse correlation has been established between the levels of DNA methylation at 5'-CCGG-3' (MspI/HpaII) sites in specific genes of integrated viral DNA in adenovirus type 12 (Ad12)-transformed hamster cell lines and the extent to which these genes are expressed ( Sutter and Doerfler , 1979, 1980). In general, early genes are transcribed into mRNA, while late genes are permanently switched off in these cell lines. Adenovirus type 2 genes methylated in vitro at 5'-CCGG-3' sites are not transcribed upon microinjection into nuclei of Xenopus laevis oocytes - unmethylated genes are expressed ( Vardimon et al., 1982a ). The MspI sites in the early and in some of the late Ad12 genes in cell lines HA12 /7, T637 , and A2497 -3 have now been precisely mapped. The data presented here reveal that the promoter/leader and 5'-regions of the early genes are unmethylated both at MspI sites and at 5'-GCGC-3' (HhaI) sites. In some instances, e.g., in the E2a regions in all three lines, the main parts of the early genes are partly methylated, even though the genes are expressed. In cell line HA12 /7, the early region E3 is not expressed, and the promoter/leader and 5'-regions of this segment are fully methylated. All late regions are completely methylated. The results suggest that the state of methylation in the promoter/leader and 5'-regions of integrated adenovirus genes is important in the control of gene expression.