Abstract
Methylation of genomic DNA is involved in the basic methanism of gene inactivation, chromatin organization, X chromosome inactivation and genomic imprinting. A pattern of DNA methylation is maintained in mitotic cells by DNA (cytosine-5) methytransferase (DNA MTase). The DNA MTase has been shown to be also expressed in postmitotic cells such as neurons. In the present report, as an approch to analyzing mechanisms underlying regulation of DNA MTase expression, we first isolated rat DNA MTase cDNA. The isolated cDNA encoded a protein of 1,622 amino acid residues showing 88.3% and 64.2% of homology with mouse and human DNA MTase, respectively. Northern blot analysis showed that DNA MTase mRNA was highly expressed in placenta during mid- to late- pregnancy. We then analyzed the expression of DNA MTase in Rcho-1 cells, a rat choriocarcinoma-derived cell line, which cease cell division but keep replicating genomic DNA when differentiated in vitro. We found that the expression of DNA MTase protein was decreased in terminally differentiated Rcho-1 cells whereas DNA MTase mRNA was consistently expressed. This result suggested posttranscriptional regulation of DNA MTase activity in Rcho-1 cells. The Rcho-1 cells would be a valuable model for studying the regulation of gene expression and function of DNA MTase in postmitotic, differentiated cells.
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