Several lines of experimental evidence indicate that DNA methylation plays a primary role in the control of gene expression and cellular differentiation in eukaryotes. At present, the regulation and specificity of the chromosomal enzyme, DNA methylase, to methylate NDA is not well understood. Recent studies have demonstrated aberrations in the levels and the stably inherited, tissue-specific pattern of genomic methylation during carcinogenesis, and have provided evidence that carcinogen adducts to DNA probably affect the sequence specific interaction and/or transmethylation reaction by DNA methylase. Since cancer is characterized by an abnormal set of expressed genes and tirggering of persistent NDA replication, a change in the pattern of genomic methylation by carcinogen binding (in particular, persistent binding) may therefore be causally related to carcinogneic initiation or evolution toward the malignant phenotype. The proposed research will focus on: 1) The regulation and enzymology of DNA methylase from normal liver and liver from various stages of hepatocarcinogenesis induced by several different carcinognes; and, 2) the elucidation and identification of factors involved in the specificity of the enzyme to recognize and bind to specific sequences and the manner in which carcinogens may alter this process. This investigation will utilize several novel methodologies to study the DNA methylation process including nick translated DNA templates to measure enzymatic activity, nitrocellulose filter binding assays and sequence defined DNA templates to study recognition site specificity, and the introduction of nick translated sequence defined templates in nuclear lysates to determine the regulation of DNA methylase. This study is aimed at determining whether DNA methylation as controlled by DNA methylase is an obligate component that might be responsible for the development of cancer.