Structural and biophysical characterization of Mycobacterium tuberculosis DciA reveals functional convergence in DnaB helicase recognition

In bacteria, DnaB replicative helicases are essential for unwinding genomic DNA to provide the single-stranded templates required for replication. The recruitment of the hexameric DnaB helicase to the origin of replication is mediated by distinct mechanisms that rely on specialized factors known as helicase loaders. The DciA family of helicase loaders exhibits distinct architectures that differ in the arrangement of the KH and DnaB-binding lasso domains. In this study, we probe the predicted structural architecture of the Mycobacterium tuberculosis (Mt) DciA helicase loader and its interplay with the cognate helicase MtDnaB. Native mass spectrometry and small-angle X-ray scattering (SAXS) analyses reveal that MtDciA adopts a monomeric and compact architecture in solution. We demonstrate that MtDnaB binds MtDciA with nanomolar affinity, likely forming a 2:1 complex at the Docking Helix–Linker Helix (DH-LH) interface of the replicative helicase. We predict that the intein-containing immature MtDnaBi will fail to form the active hexamer. As such, we suggest that MtDciA will show little to no affinity for intein-containing MtDnaBi.