Site specificity of DNA methylation and expression of herpes simplex thymidine kinase gene.

Analysis of the methylation pattern of a single-copy herpes simplex virus thymidine kinase gene integrated into the genome of mouse L cells revealed that hypomethylation of five specific AvaI sites correlates with expression of the TK gene in all of the cell lines tested. Of these specific sites, one lies 5' to the coding region, one 3' to the coding region, and three lie within the coding region of the thymidine kinase gene. Analysis of methylation at a variety of other sites using other methylation-sensitive endonucleases revealed considerable variation in the methylation patterns, apparently unrelated to gene expression and subject to variation with time in culture.