Expression of transferred thymidine kinase genes is controlled by methylation

Plasmid pTKx-1, containing the herpes simplex virus gene for thymidine kinase (TK) inserted into the BamHI site of plasmid pBR322, was introduced into Ltk^- cells by calcium phosphate precipitation in the absence of carrier DNA. Line 101 is a TK⁺ derivative of Ltk^- that contains multiple copies of pTKx-1 in a multimeric structure. A derivative of 101 that retained but no longer expressed the herpes simplex TK genes (termed 101BU1) and derivatives of line 101BU1 that reexpressed the genes (termed 101H1, 101HC, and 101HG) were selected. The TK genes in 101BU1 were hypermethylated relative to those in the TK⁺ parent and derivatives. Growth of 101BU1 in the presence of the methylation inhibitor 5-azacytidine resulted in an average 13-fold increase in the number of TK⁺ reexpressors. DNA from 101BU1 was inactive in secondary gene transfer, whereas DNA from 101 and from TK⁺ reexpressors was active. These data support a causative relationship between DNA methylation and decreased gene expression. All TK⁺ reexpressors examined had DNA rearrangements involving TK DNA.