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Nucleic Acids Research Advance Access published online on July 3, 2009
Nucleic Acids Research, doi:10.1093/nar/gkp534
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© 2009 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nucleic Acid Enzymes

The MmeI family: type II restriction–modification enzymes that employ single-strand modification for host protection

Richard D. Morgan*, Elizabeth A. Dwinell, Tanya K. Bhatia, Elizabeth M. Lang and Yvette A. Luyten

New England Biolabs, Inc. 240 County Road Ipswich, MA 01938 USA

*To whom correspondence should be addressed. Tel: 978 927 5054; Fax: 978 921 1350; Email: morgan{at}neb.com

Received March 18, 2009. Revised June 8, 2009. Accepted June 8, 2009.

The type II restriction endonucleases form one of the largest families of biochemically-characterized proteins. These endonucleases typically share little sequence similarity, except among isoschizomers that recognize the same sequence. MmeI is an unusual type II restriction endonuclease that combines endonuclease and methyltransferase activities in a single polypeptide. MmeI cuts DNA 20 bases from its recognition sequence and modifies just one DNA strand for host protection. Using MmeI as query we have identified numerous putative genes highly similar to MmeI in database sequences. We have cloned and characterized 20 of these MmeI homologs. Each cuts DNA at the same distance as MmeI and each modifies a conserved adenine on only one DNA strand for host protection. However each enzyme recognizes a unique DNA sequence, suggesting these enzymes are undergoing rapid evolution of DNA specificity. The MmeI family thus provides a rich source of novel endonucleases while affording an opportunity to observe the evolution of DNA specificity. Because the MmeI family enzymes employ modification of only one DNA strand for host protection, unlike previously described type II systems, we propose that such single-strand modification systems be classified as a new subgroup, the type IIL enzymes, for Lone strand DNA modification.


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R. D. Morgan and Y. A. Luyten
Rational engineering of type II restriction endonuclease DNA binding and cleavage specificity
Nucleic Acids Res., August 1, 2009; 37(15): 5222 - 5233.
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