Nucleic Acids Research, 2000, Vol. 28, No. 20 3962-3971
© 2000 Oxford University Press
Substrate binding in vitro and kinetics of RsrI [N6-adenine] DNA methyltransferase
Department of Biochemistry and College of Medicine, 600 South Mathews Avenue, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA and 1Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
RsrI [N6-adenine] DNA methyltransferase (M·RsrI), which recognizes GAATTC and is a member of a restriction–modification system in Rhodobacter sphaeroides, was purified to >95% homogeneity using a simplified procedure involving two ion exchange chromatographic steps. Electrophoretic gel retardation assays with purified M·RsrI were performed on unmethylated, hemimethylated, dimethylated or non-specific target DNA duplexes (25 bp) in the presence of sinefungin, a potent inhibitory analog of AdoMet. M·RsrI binding was affected by the methylation status of the DNA substrate and was enhanced by the presence of the cofactor analog. M·RsrI bound DNA substrates in the presence of sinefungin with decreasing affinities: hemimethylated > unmethylated > dimethylated >> non-specific DNA. Gel retardation studies with DNA substrates containing an abasic site substituted for the target adenine DNA provided evidence consistent with M·RsrI extruding the target base from the duplex. Consistent with such base flipping, an 
1.7-fold fluorescence intensity increase was observed upon stoichiometric addition of M·RsrI to hemimethylated DNA containing the fluorescent analog 2-aminopurine in place of the target adenine. Pre-steady-state kinetic and isotope- partitioning experiments revealed that the enzyme displays burst kinetics, confirmed the catalytic competence of the M·RsrI–AdoMet complex and eliminated the possibility of an ordered mechanism where DNA is required to bind first. The equilibrium dissociation constants for AdoMet, AdoHcy and sinefungin were determined using an intrinsic tryptophan fluorescence-quenching assay.
* To whom correspondence should be addressed. Tel: +1 217 333 2852; Fax: +1 217 333 6461; Email: gumport@uiuc.edu Present address: Sandra S. Szegedi, Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
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