Abstract
TrwB is a DNA-dependent ATPase involved in DNA transport during bacterial conjugation. The protein presents structural similarity to hexameric molecular motors such as F(1)-ATPase, FtsK, or ring helicases, suggesting that TrwB also operates as a motor, using energy released from ATP hydrolysis to pump single-stranded DNA through its central channel. In this work, we have carried out an extensive analysis with various DNA substrates to determine the preferred substrate for TrwB. Oligonucleotides with G-rich sequences forming G4 DNA structures were the optimal substrates for TrwB ATPase activity. The protein bound with 100-fold higher affinity to G4 DNA than to single-stranded DNA of the same sequence. Moreover, TrwB formed oligomeric protein complexes only with oligonucleotides presenting such a G-quadruplex DNA structure, consistent with stoichiometry of six TrwB monomers to G4 DNA, as demonstrated by gel filtration chromatography and analytical ultracentrifugation experiments. A protein-DNA complex was also formed with unstructured oligonucleotides, but the molecular mass corresponded to one monomer protein bound to one oligonucleotide molecule. Sequences capable of forming G-quadruplex structures are widespread through genomes and are thought to play a biological function in transcriptional regulation. They form stable structures that can obstruct DNA replication, requiring the action of specific helicases to resolve them. Nevertheless, TrwB displayed no G4 DNA unwinding activity. These observations are discussed in terms of a possible role for TrwB in recognizing G-quadruplex structures as loading sites on the DNA.