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Nucleic Acids Research, 1994, Vol. 22, No. 3 308-313
© 1994

MOLECULAR BIOLOGY

A mutation in helicase motif III of E.coli RecG protein abolishes branch migration of Holliday junctions

Gary J. Sharples, Matthew C. Whitby, Lizanne Ryder and Robert G. Lloyd*

Department of Genetics, University of Nottingham, Queens Medical Centre Nottingham NG7 2UH, UK

*To whom correspondence should be addressed

Received November 2, 1993. Accepted December 21, 1993.

The RecG protein of Escherichia coli catalyses branch migration of Holliday junctions made by RecA and dissociates synthetic X junctions into duplex products in reactions that require hydrolysis of ATP. To investigate the mode of action of this enzyme a chromosomal mutation that inactivates recG (recG162) was cloned and sequenced. The recG162 mutation is a G: C to A: T transition, which produces an Ala428 to Val substitution In the protein. This change affects a motif (motif III) in the protein that is highly conserved in DNA and RNA helicases. RecG162 protein was purified and shown to retain the ability to bind synthetic X and Y junctions. However, it does not dissociate these junctions and falls to catalyse branch migration of Holliday junction intermediates purified from a RecA strand exchange reaction. RecG162 retains a DNA-dependent ATPase activity, but this is much reduced relative to the wild-type protein, especially with single-stranded DNA as a co-factor. These results suggest that branch migration by RecG is related to a junction-targeted DNA helicase activity.


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