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Enzyme-dependent pausing during in vitro replication of O4-methylthymine in a defined oligonucleotide sequence
P Menichini1, M M Mroczkowska, B Singer
1Life Sciences Division, Donner Laboratory, Lawrence Berkeley Laboratory, University of California, Berkeley 94720.
Mutation Research
|May 1, 1994
Summary
O4-methylthymine (m4T) acts as a replication block for highly processive DNA polymerases like Sequenase and T7. Less processive enzymes like Klenow fragment and polymerase alpha-primase can replicate past m4T, showing enzyme nature is key.
Area of Science:
- Molecular Biology
- Biochemistry
- Enzymology
Background:
- Previous work showed O4-methylthymine (m4T) incorporation into DNA and replication by E. coli DNA polymerase I Klenow fragment (Kf).
- Replication pausing was observed before m4T with limiting enzyme concentrations, necessitating investigation into the pause mechanism (dissociation vs. stalling).
Purpose of the Study:
- To differentiate between enzyme dissociation and stalling as causes for replication pauses before m4T.
- To investigate the role of DNA polymerase type in replicating DNA containing m4T under enzyme-limiting conditions.
Main Methods:
- Utilized trapping experiments with an unmodified thymine oligonucleotide as the acceptor.
- Employed various DNA polymerases (Kf, calf-thymus polymerase alpha-primase, Sequenase, T7) under enzyme-limiting conditions.
- Performed time-dependent trapping experiments to assess enzyme binding dynamics.
Main Results:
- Enzyme type significantly influenced m4T replication; Kf and polymerase alpha-primase replicated past m4T, while Sequenase and T7 exhibited strong pauses.
- Sequenase remained bound to the template-primer irrespective of thymine modification (T or m4T).
- Kf demonstrated rapid dissociation and re-association from the template-primer.
Conclusions:
- O4-methylthymine (m4T) functions as a significant replication block for highly processive enzymes like Sequenase and T7.
- Enzymes may discriminate against incorporating m4T due to poor base pairing, leading to stalling or inactivation.
- Enzyme processivity and binding dynamics are critical factors in overcoming replication blocks caused by modified bases.