An essential transcription factor, SciP, enhances robustness of Caulobacter cell cycle regulation

A cyclical control circuit composed of four master regulators drives the Caulobacter cell cycle. We report that SciP, a helix-turn-helix transcription factor, is an essential component of this circuit. SciP is cell cycle-controlled and co-conserved with theglobal cell cycle regulator CtrA in the α-proteobacteria. SciP is expressed late inthe cell cycle and accumulates preferentially in the daughter swarmer cell. At least 58 genes, including many flagellar and chemotaxis genes, are regulated by a type 1 incoherentfeedforward motif in which CtrA activates sciP, followed by SciP repression of ctrA and CtrA target genes. We demonstrate that SciP binds to DNA at a motif distinct from the CtrAbinding motif that is present in the promoters of genes co-regulated by SciP and CtrA. SciP overexpression disrupts the balance between activation and repression of the CtrASciP coregulated genes yielding filamentous cells and loss of viability. The type 1 incoherentfeedforward circuit motif enhances the pulse-like expression of the downstream genes, andthe negative feedback to ctrA expression reduces peak CtrA accumulation. The presence of SciP in the control network enhances the robustness of the cell cycle to varying growth rates.