Mapping Protein−Protein Interactions within a Stable Complex of DNA Primase and DnaB Helicase from Bacillus stearothermophilus

For the first time, we demonstrate directly a stable complex between a bacterial DnaG (primase) and DnaB (helicase). Utilizing fragments of both proteins, we are able to dissect interactions within this complex and provide direct evidence that it is the C-terminal domain of primase that interacts with DnaB. Furthermore, this C-terminal domain is sufficient to induce maximal stimulation of the helicase and ATPase activities of DnaB. However, the region of DnaB that interacts with the C-terminal domain of primase appears to comprise a surface on DnaB that includes regions from both of the previously identified N- and C-terminal domains. Using a combination of biochemical and physical techniques, we show that the helicase-primase complex comprises one DnaB hexamer and either two or three molecules of DnaG. Our results show that in Bacillus stearothermophilus the helicase-primase interaction at the replication fork may not be transient, as was shown to be the case in Escherichia coli. Instead, primase appears to interact with the helicase forming a tighter complex with enhanced ATPase and helicase activities.