Bacterial replication initiation as precision control in biology

Replication initiation in bacteria is one of the most precise physiological controls in biology. Curiously, E. coli produces more than one order of magnitude larger number of DnaA proteins than needed, the widely conserved master regulator proteins in bacteria. More than 90% of the DnaA proteins are titrated by 300 binding sites encoded in the chromosome known as the DnaA boxes. In this work, we demonstrate that the number of DnaA boxes is optimal for initiation noise reduction and far more effective than autoregulation of DnaA production. We developed several novel experimental methods to control the number of DnaA boxes. As we increase the number of DnaA boxes, initiation is delayed steadily. However, the initiation noise (measured by CV) shows a non-monotonic behavior with the minimum at the wild-type number. We propose a model based on the DnaA flux to explain our results. We further discuss general implications of our results for precision control in biology.