Functional characterization of DnaB helicase and its modulation by single-stranded DNA binding protein in...

DnaB is important in the initiation and extension stages of DNA replication. Although DnaB has been studied in many bacterial species, its function in the devastating human pathogen Mycobacterium tuberculosis remains unclear. In this study, an intein-deleted form of M. tuberculosis DnaB (MtbDnaB) was cloned, expressed and characterized. MtbDnaB exhibited strong 5' to 3' helicase and ATPase activities, suggesting that MtbDnaB is a functional homolog of Escherichia coli DnaB. A physical interaction between MtbSSB (single-stranded binding protein of M. tuberculosis) and MtbDnaB was further identified in vivo and in vitro. The MtbSSB C-terminal fragment was found to have a critical function in this interaction. Moreover, the helicase activity of MtbDnaB was stimulated by MtbSSB at low concentrations and inhibited at high concentrations. An MtbSSB mutant with decreased binding affinity for ssDNA can stimulate the helicase activity of MtbDnaB over a wider concentration range than wild-type MtbSSB. These results suggest that MtbSSB assists in the loading of MtbDnaB on the DNA replication fork in M. tuberculosis.