Cell size regulation in bacteria: a tale of old regulators with new mechanisms

Successful proliferation of bacteria requires the coordination of multiple cellular events. The precise coordination between the replication of the chromosome with cell growth and division is essential for daughter cells to inherit an intact copy of the genome with the appropriate cell size. However, the molecular mechanisms that temporally and spatially integrate these essential processes have remained elusive in the field. Using the model system C. crescentus, we uncovered a previously unrecognized role for the conserved chromosome replication initiator DnaA in regulating cell growth and division dynamics. Through mutant analyses, transcriptional profiling, biochemical and high-resolution imaging, our data demonstrate that DnaA modulates cell size through direct transcriptional activation of enzymes involved in cell wall biosynthesis. We identify a conserved cell wall biosynthetic enzyme, MurD, as a key downstream effector and demonstrate its unexpected ability to restrict cell size. Both in vivo and in vitro analyses revealed that the transcriptional regulation of murD requires the ATP-bound form of DnaA, the same nucleotide-bound form required for initiating chromosome replication. These findings highlight how bacteria leverage the conserved nucleotide-dependent switch, DnaA-ATP, to temporally orchestrate spatially distinct processes ensuring accurate progression of the cell cycle.