Abstract
Besides modulating specific DNA-protein interactions, methylated cytosine, frequently referred to as the fifth base of the genome, also influences DNA structure, recombination, transposition, repair, transcription, imprinting, and mutagenesis. DNA (cytosine-5-)-methyltransferase catalyzes cytosine methylation in eukaryotes. We have cloned and expressed this enzyme in Escherichia coli, purified it to apparent homogeneity, characterized its properties, and we have shown that it hemimethylates DNA. The cDNA for murine maintenance methyltransferase was reconstructed and cloned for direct expression in native form. Immunoblotting revealed a unique protein (M(r) = 190,000) not present in control cells. The mostly soluble overexpressed protein was purified by DEAE, Sephadex, and DNA cellulose chromatography. Peak methylating activity correlated with methyltransferase immunoblots. The purified enzyme preferentially transferred radioactive methyl moieties to hemimethylated DNA in assays and on autoradiograms. All of the examined properties of the purified recombinant DNA methyltransferase are consistent with the enzyme purified from mammalian cells. Further characterization revealed enhanced in vitro methylation of premethylated oligodeoxynucleotides. The cloning of hemimethyltransferase in E. coli should allow facilitated structure-function mutational analysis of this enzyme, studies of its biological effects in prokaryotes, and potential large scale methyltransferase production for crystallography, and it may have broad applications in maintaining the native methylated state of cloned DNA.
Highlights
Besides modulating specific DNA-protein interactions, methylated cytosine, frequently referred to as the fifth base of the genome, influences DNA structure, recombination, transposition, repair, transcription, imprinting, and mutagenesis
Mammalian DNA (Cytosine-5-)-methyltransferase Expressed in Escherichia coli, Purified and Characterized*
We have cloned and expressed this enzyme in Escherichia coli, purified it to apparent homogeneity, characterized its properties, and we have shown that it hemimethylates DNA
Summary
Vol 270, No 31, 1ssue of August 4, pp. 18543-18550, 1995 Printed in U.S.A. Mammalian DNA (Cytosine-5-)-methyltransferase Expressed in Escherichia coli, Purified and Characterized*. After semiconservative replication of DNA, both daughter duplexes are hemimethylated, and DNA MTase, which is localized to replication foci [27], fully methylates the duplex CpG dinucleotides This process, termed maintenance methylation, restores the parental genomic methylation pattern and is consistent with the in vitro propensity of the DNA MTase for hemimethylated sequences [28,29]. Studies of the effects of the recombinant methyltransferase on the prokaryotic genome and cellular processes will be useful in further elucidating the biological significance of DNA methylation.
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