A DNA translocation motif in the bacterial transcription--repair coupling factor, Mfd

The bacterial transcription-repair coupling factor, Mfd, is a superfamily II helicase that releases transcription elongation complexes stalled by DNA damage or other obstacles. Transcription complex displacement is an ATP-dependent reaction that is thought to involve DNA translocation without the strand separation associated with classical helicase activity. We have identified single amino acid substitutions within Mfd that disrupt the ability of Mfd to displace RNA polymerase but do not prevent ATP hydrolysis or binding to DNA. These substitutions, or deletion of the C-terminal 209 residues of Mfd, abrogate the ability of Mfd to increase the efficiency of roadblock repression in vivo. The substitutions fall in a region of Mfd that is homologous to the 'TRG' motif of RecG, a protein that catalyses ATP-dependent translocation of Holliday junctions. Our results define a translocation motif in Mfd and suggest that Mfd and RecG couple ATP hydrolysis to translocation of DNA in a similar manner.