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J. Biol. Chem., Vol. 279, Issue 49, 51568-51573, December 3, 2004

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Prereplicative Purine Methylation and Postreplicative Demethylation in Each DNA Duplication of the Escherichia coli Replication Cycle^*

Renata Maas

From the Department of Microbiology, New York University School of Medicine, New York, New York 10016

Escherichia coli plasmid DNA activated for initiation of duplication is in a stable low linking number supercoiled conformation. Low linking number DNA is methylated at the internal purines of a frequent 5'-Pyr-Pyr-Pur-Pur tetramer with a 5'-Pyr-Pur-3' axis of symmetry and is cut at the axis of symmetry by pneumococcal restriction enzyme DpnI when methylated in both strands. Purine methylation is of adenine in one strand and guanine in the other. Methylation of one of the two purines is removed during the cell cycle, presumably before the reverse shift to the B-supercoiled conformation. The topological transition was reconstituted in vitro only with DNA unmethylated at purines. Methylation-restriction analyses coupled with the chemical properties of low-linking number DNA and B-DNA respectively, suggest that removal of guanine methylation is essential for the low-linking number to B-DNA transition and hence for the deactivation of replication. Demethylation of methylguanine could explain the presence in E. coli of the two-member inducible operon known as ada. Characteristics of ada suggest a cascade of chemical DNA modifications that reverse prereplicative guanine methylation. Guanine demethylation could provide a model for the pivotal role played by de novo methylation in replication and for the essential role of "repair" enzyme ExoIII in demethylation leading to the reversal of replicative DNA activation and other processes that affect DNA function.

Received for publication, July 1, 2004 , and in revised form, August 31, 2004.

The nucleotide sequence(s) reported in this paper has been submitted to the DDBJ/GenBank^TM/EBI Data Bank with accession number(s) M16167.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Microbiology, New York University School of Medicine, 550 First Ave., New York, NY 10016. E-mail: maasr01{at}endeavor.med.nyu.edu.

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