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Altering the sequence specificity of HaeIII methyltransferase by directed evolution using in vitro
Helen M Cohen1, Dan S Tawfik, Andrew D Griffiths
1MRC Centre for Protein Engineering and MRC Laboratory for Molecular Biology, Cambridge CB2 2QH, UK.
Protein Engineering, Design & Selection : PEDS
|February 27, 2004
Summary
Researchers engineered a DNA methyltransferase enzyme using directed evolution to efficiently methylate a new DNA sequence. This novel enzyme variant shows improved catalytic efficiency, surpassing the wild-type enzyme for its primary substrate.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzyme Engineering
Background:
- Engineering DNA-modifying enzyme specificity is challenging due to poor understanding of sequence recognition.
- Wild-type HaeIII methyltransferase (M.HaeIII) methylates GGCC but also promiscuously methylates non-canonical sites like AGCC at lower rates.
Purpose of the Study:
- To engineer a variant of M.HaeIII methyltransferase with enhanced specificity and efficiency for a novel target DNA site.
- To improve the enzyme's ability to methylate the AGCC sequence.
Main Methods:
- Directed evolution using in vitro compartmentalization (IVC) to select for M.HaeIII variants that efficiently methylate AGCC.
- A two-step mutagenesis strategy targeting DNA-contacting residues and a nearby loop.
Main Results:
- A mutant M.HaeIII enzyme was generated with a 670-fold improvement in catalytic efficiency (k(cat)/K(m)(DNA)) for AGCC.
- The evolved mutant exhibits a preference for AGCC over the canonical GGCC site.
- The mutant efficiently methylates AGCC, CGCC, and GGCC, while maintaining discrimination against other non-canonical sites like TGCC.
Conclusions:
- Directed evolution can successfully engineer DNA-modifying enzymes with altered specificity and improved catalytic efficiency.
- The evolved M.HaeIII mutant demonstrates superior performance with a non-canonical substrate compared to the wild-type enzyme.
- This study presents a rare case where a laboratory-evolved enzyme's efficiency exceeds that of the wild-type for its principal substrate.