Type III restriction enzymes need two inversely oriented recognition sites for DNA cleavage
A Meisel1, T A Bickle, D H Krüger
1Institute of Virology, Humboldt University Medical School, Charité, Berlin, Germany.
Nature
|January 30, 1992
Summary
Type III restriction enzymes avoid self-destruction by requiring two recognition sites in inverse orientation. Newly replicated DNA, with sites in the same orientation, is thus protected from restriction, clarifying a long-standing paradox.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Type III restriction/modification enzymes target specific DNA sequences.
- These enzymes methylate only one DNA strand, leading to hemimethylated DNA after replication.
- Unmodified sites in newly replicated DNA paradoxically escape restriction.
Purpose of the Study:
- To elucidate the mechanism preventing restriction of unmodified DNA sites after replication.
- To understand the orientation and spacing requirements for Type III enzyme activity.
Main Methods:
- Investigated the orientation and distance requirements for Type III restriction.
- Analyzed DNA replication intermediates and enzyme activity.
Main Results:
- Restriction by Type III enzymes necessitates two unmodified recognition sites.
- These sites must be in an inverse orientation, irrespective of distance.
- Newly replicated DNA contains unmodified sites in the same orientation, explaining their lack of restriction.
Conclusions:
- The inverse orientation requirement explains why newly replicated DNA is not restricted.
- This finding offers insights into DNA strand-specific modifications and anisotropy.
- Potential relevance to phenomena like genetic imprinting.
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