Methylation of a plasmid containing the sv4O promoter linked to the chloramphenicol acetyl transferase (CAT) gene, with either murine DNA methylase or methylase 5ssl results in inhibition of the expression or the reporter gene after transfection into cultured cells. Methylation of the plasmid with the methylases HhaI and HpaII has no effect on the expression of this gene. Protein-DNA interactions in the SV40 promoter are not affected by the presence of methylcytosine suggesting that inactivation results from the formation of an inactive chromatin structure that is dependent on the high CG content of the plasmid.
