One-codon alternative splicing of the CpG MTase Dnmt1 transcript in mouse somatic cells

The genomic methylation patterns in the mammalian somatic cells are presuma bly maintained by a single enzyme, dnmt1, In mouse, this DNA (cytosine-5)-m ethyltransferase, or CpG MTase, is encoded by the Dnmt1 gene. We now presen t evidence that in different tissues and cell types, the primary transcript of mouse dnmt1 is alternatively spliced to generate two poly-(A) RNAs of a pproximately similar abundance, This alternative splicing most likely origi nates from the existence of two tandemly arranged acceptor sites separated by only 3 nt. The two Dnmt1 mRNAs thus encode two CpG MTases differing by t wo amino acids, We discuss the implications of the discovery of two dnmt1 i sozymes, instead of one enzyme as previously thought, in the somatic cells of both mouse and human. (C) 2000 Federation of European Biochemical Societ ies.