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 methyl transferases that lacks a cognate endonuclease. This endogenous cell cycle regulated adenine DNA methyltransferase (CcrM) is essential for C, crescent us cellular viability. DNA methylation catalyzed by CcrM, provides an oblig atory signal for the proper progression through the cell cycle. To further our understanding of the regulatory role played by CcrM, we sought to inves tigate its biophysical properties, In this paper we employed equilibrium ul tracentrifugation, velocity ultracentrifugation, and chemical cross-linking to show that CcrM is dimeric at physiological concentrations. However, sur face plasmon resonance experiments in the presence of S-adenosyl-homocystei ne evince that CcrM binds as a monomer to a defined hemi-methylated DNA sub strate containing the canonical methylation sequence, GANTC, Initial veloci ty experiments demonstrate that dimerization of CcrM does not affect DNA me thylation. Collectively, these findings suggest that CcrM is active as a mo nomer and provides a possible in vivo role for dimerization as a means to s tabilize CcrM from premature catabolism.