Nucleic Acids Research Advance Access published online on October 17, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn711
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Nucliec Acid Enzymes |
MmeI: a minimal Type II restriction-modification system that only modifies one DNA strand for host protection
1New England Biolabs Inc., Ipswich, MA and 2MCB Department, Brown University, Providence, RI, USA
*To whom correspondence should be addressed. Tel: +1 978 927 5054 ext. 7270; Fax: +1 978 921 1350; Email: morgan{at}neb.com
Received June 6, 2008. Revised September 26, 2008. Accepted September 30, 2008.
MmeI is an unusual Type II restriction enzyme that is useful for generating long sequence tags. We have cloned the MmeI restriction-modification (R-M) system and found it to consist of a single protein having both endonuclease and DNA methyltransferase activities. The protein comprises an amino-terminal endonuclease domain, a central DNA methyltransferase domain and C-terminal DNA recognition domain. The endonuclease cuts the two DNA strands at one site simultaneously, with enzyme bound at two sites interacting to accomplish scission. Cleavage occurs more rapidly than methyl transfer on unmodified DNA. MmeI modifies only the adenine in the top strand, 5'-TCCRAC-3'. MmeI endonuclease activity is blocked by this top strand adenine methylation and is unaffected by methylation of the adenine in the complementary strand, 5'-GTYGGA-3'. There is no additional DNA modification associated with the MmeI R-M system, as is required for previously characterized Type IIG R-M systems. The MmeI R-M system thus uses modification on only one of the two DNA strands for host protection. The MmeI architecture represents a minimal approach to assembling a restriction-modification system wherein a single DNA recognition domain targets both the endonuclease and DNA methyltransferase activities.
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