The hexameric E-coli DnaB helicase can exist in different quarternary states

The DnaB protein is the primary replicative helicase in Escherichia coli, and the active form of the protein is a hexamer. It has been reported that the protein forms a ring with strong 3-fold symmetry, which was suggested to be a trimer of dimers. We show that under different conditions, using either ATP, ATP gamma S, AMP-PNP or ADP as nucleotide cofactors, we always find two different forms of the DnaB ring; one with a 3-fold symmetry and one with 6-fold symmetry We have used scanning transmission electron microscopy for mass analysis, and have found that both forms are hexamers, excluding the possiblity that the 3-fold form is in fact a trimer of the 52 kDa monomer. We have also found rings that are in an intermediate state between these two. The existence of hexamers in discrete states shows that the transitions between these states must be cooperative. These observations suggest that there may be an equilibrium between two different conformations of the hexameric ring. The role of these two states in the mechanism of helicase action remains to be determined. (C) 1996 Academic Press Limited.