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Updated: Jun 21, 2026
Molecular Evolution of the Tre Recombinase
Published on: May 29, 2008
A molecular throttle: the recombination hotspot chi controls DNA translocation by the RecBCD helicase
Maria Spies1, Piero R Bianco, Mark S Dillingham
1Section of Microbiology, Center for Genetics and Development, University of California, Davis 9561, USA.
Insights
The RecBCD enzyme, crucial for DNA repair, pauses at the chi sequence. This interaction slows its DNA translocation speed by half, suggesting a motor subunit uncoupling.
Area of Science:
- Molecular Biology
- Biochemistry
- Genetics
Background:
- RecBCD enzyme is a key helicase/nuclease initiating homologous recombination at DNA breaks.
- The DNA sequence chi regulates RecBCD enzyme's nuclease activity and RecA loading.
- Chi recognition influences RecBCD enzyme's interaction with DNA.
Purpose of the Study:
- To investigate the effect of the chi sequence on RecBCD enzyme translocation dynamics.
- To elucidate the mechanism by which chi influences RecBCD enzyme activity.
Main Methods:
- Single-molecule DNA translocation assays were used to observe individual RecBCD enzyme movements.
- Real-time observation of RecBCD enzyme translocation along DNA molecules.
Main Results:
- RecBCD enzyme was observed to pause precisely at the chi DNA sequence.
- Following the pause at chi, RecBCD enzyme's translocation rate decreased by approximately 50% of its initial speed.
- This suggests a change in the enzyme's motor function upon chi interaction.
Conclusions:
- The chi sequence modulates RecBCD enzyme translocation, causing a significant pause and subsequent rate reduction.
- This modulation is proposed to result from the uncoupling of a motor subunit, likely RecD, from the holoenzyme.
- This mechanism provides new insights into the regulation of DNA repair and recombination initiation.
Abstract:
RecBCD enzyme is a heterotrimeric helicase/nuclease that initiates homologous recombination at double-stranded DNA breaks. Several of its activities are regulated by the DNA sequence chi (5'-GCTGGTGG-3'), which is recognized in cis by the translocating enzyme. When RecBCD enzyme encounters chi, the intensity and polarity of its nuclease activity are changed, and the enzyme gains the ability to load RecA protein onto the chi-containing, unwound single-stranded DNA. Here, we show that interaction with chi also affects translocation by RecBCD enzyme. By observing translocation of individual enzymes along single molecules of DNA, we could see RecBCD enzyme pause precisely at chi. Furthermore, and more unexpectedly, after pausing at chi, the enzyme continues translocating but at approximately one-half the initial rate. We propose that interaction with chi results in an enzyme in which one of the two motor subunits, likely the RecD motor, is uncoupled from the holoenzyme to produce the slower translocase.
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