A Cell-Type-Specific Protein-Protein Interaction Modulates Transcriptional Activity of a Master Regulator in Caulobacter crescentus

Abstract

Summary Progression through the Caulobacter cell cycle is driven by the master regulator CtrA, an essential two-component signaling protein that regulates the expression of nearly 100 genes. CtrA is abundant throughout the cell cycle except immediately prior to DNA replication. However, the expression of CtrA-activated genes is generally restricted to S phase. We identify the conserved protein SciP (small CtrA inhibitory protein) and show that it accumulates during G1, where it inhibits CtrA from activating target genes. The depletion of SciP from G1 cells leads to the inappropriate induction of CtrA-activated genes and, consequently, a disruption of the cell cycle. Conversely, the ectopic synthesis of SciP is sufficient to inhibit CtrA-dependent transcription, also disrupting the cell cycle. SciP binds directly to CtrA without affecting stability or phosphorylation; instead, SciP likely prevents CtrA from recruiting RNA polymerase. CtrA is thus tightly regulated by a protein-protein interaction which is critical to cell-cycle progression.

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

“…S3), suggesting that increased proteolysis was not the reason for the decreased level of CtrA during sciP over production. We found, as did Gora et al (14), that CtrA was stabilized during sciP overexpression (Fig. S4).…”

“…CtrA activates the transcription of sciP and SciP negatively regulates CtrA accumulation by binding to the promoter of ctrA and repressing its transcription. Our coimmunoprecipitation results and reported yeast two-hybrid and gel retardation experiments (14) suggest that interaction between SciP and CtrA mediates SciP repression of CtrA activated genes. We propose that SciP's principal function in cell cycle regulation is to enhance the robustness of the core cell cycle control circuit.…”

“…5B), strongly suggesting that the regulatory action of DNA-bound SciP causes the phenotype. Two of these substitutions (R35A and R40A) were also identified by Gora et al (14) and interpreted as being in the site involved in a CtrA-SciP interaction.…”

“…To control the timing of CtrA function, the cell regulates both CtrA activity, by controlling its phosphorylation state, and CtrA accumulation, by inhibiting ctrA transcription upon DNA remethylation of newly replicated chromosomes (11), and by the temporal regulation of CtrA proteolysis (12,13). We report here that SciP (CC0903), also identified by Gora et al (14), provides another layer of control. SciP represses the transcription of late predivisional cell and swarmer cell genes that were activated by CtrA.…”

“…The evidence reported in this article and results of Gora et al (14) show that SciP acts as a repressor of genes activated by CtrA. Gora et al (14) surmised that the mechanism of this repression involved SciP binding solely to CtrA and thus preventing CtrA from recruiting RNA polymerase. We have discovered that SciP, a winged HTH protein, binds directly to a specific motif present in the promoters of genes activated by CtrA and repressed by SciP.…”