The N-terminus promotes oligomerization of the Escherichia coli initiator protein DnaA.

Initiation of chromosome replication in Escherichia coli is governed by the interaction of the initiator protein DnaA with the replication origin oriC. Here we present evidence that homo-oligomerization of DnaA via its N-terminus (amino acid residues 1-86) is also essential for initiation. Results from solid-phase protein-binding assays indicate that residues 1-86 (or 1-77) of DnaA are necessary and sufficient for self interaction. Using a 'one-hybrid-system' we found that the DnaA N-terminus can functionally replace the dimerization domain of coliphage lambda cl repressor: a lambdacl-DnaA chimeric protein inhibits lambda plasmid replication as efficiently as lambdacI repressor. DnaA derivatives with deletions in the N-terminus are incapable of supporting chromosome replication from oriC, and, conversely, overexpression of the DnaA N-terminus inhibits initiation in vivo. Together, these results indicate that (i) oligomerization of DnaA N-termini is essential for protein function during initiation, and (ii) oligomerization does not require intramolecular cross-talk with the nucleotide-binding domain III or the DNA-binding domain IV. We propose that E. coli DnaA is composed of largely independent domains - or modules - each contributing a partial, though essential, function to the proper functioning of the 'holoprotein'.