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

Initiation of chromosome replication in Escherichia coil 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-term inus (amino acid residues 1-86) is also essential for initiation. Results f rom 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-h ybrid-system' we found that the DnaA N-terminus can functionally replace th e dimerization domain of coliphage lambda cl repressor: a lambda cl-DnaA ch imeric protein inhibits lambda plasmid replication as efficiently as lambda cl repressor. DnaA derivatives with deletions in the N-terminus are incapa ble of supporting chromosome replication from oriC, and, conversely, overex pression of the DnaA N-terminus inhibits initiation in vivo. Together, thes e results indicate that (i) oligomerization of DnaA N-termini is essential for protein function during initiation, and (ii) oligomerization does not r equire intramolecular cross-talk with the nucleotide-binding domain III or the DNA-binding domain IV. We propose that E. coli DnaA is composed of larg ely independent domains - or modules - each contributing a partial, though essential, function to the proper functioning of the 'holoprotein'.