Proper spatial and temporal regulation of the cell cycle is crucial to the survival and proliferation of all organisms. Complex regulatory circuits have evolved to ensure stable, orderly progression through the cell cycle. Surprisingly, the molecular mechanisms and design principles underlying these circuits remain incompletely understood, particularly in bacteria. The alpha-proteobacterium Caulobacter crescentus is an experimentally tractable system for elucidating the fundamental mechanisms underlying cell cycle regulation in bacteria, and for understanding regulation at the systems level. The Caulobacter cell cycle is driven largely by two essential regulators: CtrA and DnaA. CtrA is a response regulator and two-component signal transduction protein that directly regulates the expression of nearly 100 genes, many of which coordinate cell division. CtrA also directly binds to and silences the origin of replication in daughter swarmer cells, but is inactivated and degraded in daughter stalked cells. CtrA thus governs the asymmetric replicative fates of daughter cells, a hallmark of the Caulobacter cell cycle. However, CtrA does not significantly influence the fundamental periodicity of DNA replication. Instead, DnaA, a nearly universal bacterial protein that initiates DNA replication, governs the periodicity of replication. In rich media, DnaA protein levels are constant, indicating that it is regulated predominantly at the level of activity. Although substantial progress has been made in understanding the regulation of CtrA and, to a lesser extent, DnaA, major gaps remain. The goal of this project is to define the complete molecular circuitry controlling CtrA and DnaA in both nutrient replete conditions and following starvation.
We aim to identify new components of this circuitry, to elucidate their connections to DnaA and CtrA, and to understand their dynamics using a combination of genetic, biochemical, bioinformatic, genomic, and cell biological approaches. Specifically, we aim to (i) determine how the activation of CtrA is coupled to DNA replication initiation, (ii) idenify and characterize factors that regulate DnaA activity during growth in rich media, and (iii) elucidate the molecular mechanisms governing CtrA and DnaA activity following nutrient starvation. These studies will unveil how bacterial cells tightly regulate DNA replication and modulate cell cycle progression in a range of growth conditions. A better understanding of how bacteria regulate their cell cycle will guide the development of new antibiotics that slow or halt the proliferation of pathogens. Finally, by comparing the regulatory strategies unveiled here to those employed in eukaryotes our work will help to reveal the design principles of regulatory circuits throughout biology.

Public Health Relevance

This project will aid efforts to develop new antibiotics by identifying and understanding essential components of the cell cycle machinery in bacteria. We have a specific focus on two-component signal transduction proteins, which have emerged as attractive drug targets because they are critical to the proliferation of bacteria but conspicuousl absent from metazoans. We are also focused on understanding DnaA, a nearly universal regulator of DNA replication in bacteria that is also, in principle, an ideal target for new antibiotics.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Prokaryotic Cell and Molecular Biology Study Section (PCMB)
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Hamlet, Michelle R
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Massachusetts Institute of Technology
Schools of Arts and Sciences
United States
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Guo, Monica S; Haakonsen, Diane L; Zeng, Wenjie et al. (2018) A Bacterial Chromosome Structuring Protein Binds Overtwisted DNA to Stimulate Type II Topoisomerases and Enable DNA Replication. Cell 175:583-597.e23
Tran, Ngat T; Laub, Michael T; Le, Tung B K (2017) SMC Progressively Aligns Chromosomal Arms in Caulobacter crescentus but Is Antagonized by Convergent Transcription. Cell Rep 20:2057-2071
García-Bayona, Leonor; Guo, Monica S; Laub, Michael T (2017) Contact-dependent killing by Caulobacter crescentus via cell surface-associated, glycine zipper proteins. Elife 6:
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Le, Tung Bk; Laub, Michael T (2016) Transcription rate and transcript length drive formation of chromosomal interaction domain boundaries. EMBO J 35:1582-95
Aakre, Christopher D; Herrou, Julien; Phung, Tuyen N et al. (2015) Evolving new protein-protein interaction specificity through promiscuous intermediates. Cell 163:594-606
Badrinarayanan, Anjana; Le, Tung B K; Laub, Michael T (2015) Rapid pairing and resegregation of distant homologous loci enables double-strand break repair in bacteria. J Cell Biol 210:385-400

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