Disrupting NtrC function reveals unexpected robustness in a central cell cycle regulatory network
Hunter North1, Molly Hydorn2, Jonathan Dworkin2
1Department of Microbiology, Genetics and Immunology, Michigan State University, East Lansing, Michigan, USA.
Mbio
|August 18, 2025
Summary
Mutations in the nitrogen assimilation regulator NtrC can bypass the essential CckA cell cycle kinase in Caulobacter. This bypass involves metabolic shifts and altered gene expression, revealing the robustness of cell cycle control.
Area of Science:
- Microbiology
- Cell Biology
- Molecular Biology
Background:
- Cell cycle progression depends on signaling pathways integrating cellular and environmental cues.
- In Alphaproteobacteria, the CckA histidine kinase regulates the master cell cycle regulator CtrA.
- Understanding connections between CckA signaling and other cellular processes is crucial.
Purpose of the Study:
- To identify mutations bypassing the loss of essential CckA function in Caulobacter.
- To investigate the role of the nitrogen assimilation regulator NtrC in suppressing cckA defects.
- To elucidate the mechanisms by which NtrC mutations rescue cell viability and alter gene expression.
Main Methods:
- Selection for mutations bypassing temperature-sensitive cckA mutants.
- Genetic analysis of loss-of-function ntrC alleles.
- Measurement of intracellular glutamine and ppGpp levels.
- Assessment of CtrA protein stability and transcriptional activity of sigma54-dependent genes.
Main Results:
- Loss-of-function ntrC mutations differentially suppressed cckA(ts) viability defects.
- Complete NtrC loss reduced glutamine, increased ppGpp, sustained CtrA, and partially rescued viability.
- NtrC mutants lacking DNA-binding domains fully rescued viability and activated a subset of flagellar genes independently of the canonical GAFTGA motif.
Conclusions:
- Bypass of CckA function by NtrC mutants involves metabolic changes and emergent transcriptional activity.
- NtrC DNA-binding mutants modulate transcription from non-native sites, suggesting novel regulatory roles.
- This study highlights the mutational robustness of the Caulobacter cell cycle control system and evolutionary adaptability.
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