Identification of the active oligomeric state of an essential adenine DNA methyltransferase from Caulobacter crescentus

Caulobacter crescentus contains one of the two known prokaryotic DNA methyltransferases that lacks a cognate endonuclease. This endogenous cell cycle regulated adenine DNA methyltransferase (CcrM) is essential for C. crescentus cellular viability. DNA methylation catalyzed by CcrM provides an obligatory signal for the proper progression through the cell cycle. To further our understanding of the regulatory role played by CcrM, we sought to investigate its biophysical properties. In this paper we employed equilibrium ultracentrifugation, velocity ultracentrifugation, and chemical cross-linking to show that CcrM is dimeric at physiological concentrations. However, surface plasmon resonance experiments in the presence of S-adenosyl-homocysteine evince that CcrM binds as a monomer to a defined hemi-methylated DNA substrate containing the canonical methylation sequence, GANTC. Initial velocity experiments demonstrate that dimerization of CcrM does not affect DNA methylation. Collectively, these findings suggest that CcrM is active as a monomer and provides a possible in vivo role for dimerization as a means to stabilize CcrM from premature catabolism.