Mutational analysis of conserved hydrophobic amino acid residues in the N-terminal region of DnaA protein

DnaA is the initiator of chromosomal DNA replication in E. coli. We previously reported that conserved hydrophobic amino acid residues in the N-terminal region of DnaA (I26 and L40) are essential for DNA replication in vivo and in vitro using mutant DnaA proteins (DnaAI26S and DnaAL40S). In this study, we introduced further random mutations to find intragenic suppressors for dnaAI26S or dnaAL40S. By direct DNA sequence, a mutation which causes substitution of the Ser (Ile, in the wild-type DnaA) with Phe (DnaAI26F or DnaAL40F) was found in all of the suppressors. Site-directed mutational analysis showed that DnaAI26L, and DnaAL40I, but not DnaAI26S or DnaAL40S, were active for oriC DNA replication in cells. Furthermore, purified DnaAI26F but not DnaAI26S was active for oriC DNA replication in a crude extract. These results strongly suggest that hydrophobic amino acid residues in these positions of DnaA (I26 and L40) are important for the function of this protein as an initiator of DNA replication both in vivo and in vitro.