THE ESCHERICHIA-COLI RUVB BRANCH MIGRATION PROTEIN FORMS DOUBLE HEXAMERIC RINGS AROUND DNA

The RuvB protein is induced in Escherichia coil as part of the SOS res ponse to DNA damage. It is required for genetic recombination and the postreplication repair of DNA. In vitro, the RuvB protein promotes the branch migration of Holliday junctions and has a DNA helicase activit y in reactions that require ATP hydrolysis. We have used electron micr oscopy, image analysis, and three-dimensional reconstruction to show t hat the RuvB protein, in the presence of ATP, forms a dodecamer on dou ble-stranded DNA in which two stacked hexameric rings encircle the DNA and are oriented in opposite directions with D-6 symmetry. Although h elicases are ubiquitous and essential for many aspects of DNA repair, replication, and transcription, three-dimensional reconstruction of a helicase has not yet been reported, to our knowledge. The structural a rrangement that is seen may be common to other helicases, such as the simian virus 40 large tumor antigen.