Determination of the secondary structure in solution of the Escherichia coli DnaA DNA-binding domain

DnaA protein binds specifically to a group of binding sites collectively called as DnaA boxes within the bacterial replication origin to induce local unwinding of duplex DNA. The DNA-binding domain of DnaA, domain IV, comprises the C-terminal 94 amino acid residues of the protein. We overproduced and purified a protein containing only this domain plus a methionine residue. This protein was stable as a monomer and maintained DnaA box-specific binding activity. We then analyzed its solution structure by CD spectrum and heteronuclear multi-dimensional NMR experiments. We established extensive assignments of the ¹H, ¹³C, and ¹⁵N nuclei, and revealed by obtaining combined analyses of chemical shift index and NOE connectivities that DnaA domain IV contains six α-helices and no β-sheets, consistent with results of CD analysis. Mutations known to reduce DnaA box-binding activity were specifically located in or near two of the α-helices. These findings indicate that the DNA-binding fold of DnaA domain IV is unique among origin-binding proteins.