Magic spot coordinates fat availability with the bacterial cell division cycle

EMBARGOED UNTIL: Monday 5/20, 3 PM MDT

(Symposium Session 141, Paper )

Shannon Wood
California State University Northridge
Valencia, CA, United States
Phone: (818)677-4478
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Northridge researchers discovered that bacteria starved for fat require magic spot (ppGpp) for the synthesis of a cell cycle control protein that blocks DNA replication under inappropriate conditions.  Bacteria need to synthesize all of their cellular components to reproduce, so that each cell division cycle produces a new bacterial cell.  While much is known about the proteins required for bacteria to divide (reproduce), less is known about how they exit their reproductive cycle when a cell lacks enough of at least one cellular component to create a new bacterial cell.  In nature, most microorganisms are starving – they don’t have the rich nutrients generously provided to them in laboratory media to promote rapid growth.  The study of how bacteria survive bacteriostatic (non-growing) conditions, for instance, when they are exposed to certain antibiotics, could facilitate development of drugs to work with bacteriostatic antibiotics to block the processes needed for bacteria to re-enter the cell cycle.

Aquatic bacteria, such as Caulobacter crescentus, spend most of their time in the environment under nutrient-limiting conditions.  Thus, Caulobacter is a model organism for studying both how cells enter and exit the bacterial cell division (or reproductive) cycle.  Work from Professor Sean Murray’s microbiology laboratory at California State University Northridge critically shows that fat biosynthesis is a cell cycle checkpoint in Caulobacter.  Caulobacter detects when there is not enough fat to produce the cell membrane (a barrier between the inside and outside of the cell) and consequently exits the cell cycle.  To exit the cell cycle in a way that allows Caulobacter to re-enter it again when fat supplies are replenished, a magic spot (ppGpp) ensures the synthesis of a protein that blocks DNA replication until the cell can synthesize all of the components needed to create a new cell.  Their work shows that this magic spot allows cells to survive exiting the cell cycle (bacteriostasis, a condition similar to hibernation) by blocking inappropriate DNA replication and an increase in cell size. In the absence of this magic spot, cells cannot survive prolonged bacteriostasis (hibernation) and can no longer divide to create new cells, even if the ability to synthesize fat is restored prior to cell death.

This work was performed by Shannon Wood, a current biology graduate student, and Kristina Stott, who completed the graduate program in May 2012, along with undergraduate researchers Bao Nguyen, Anabel Herrera, and Yannet Perez Mora. Both Shannon and Kristina were granted graduate fellowships by the Interdisciplinary Research Institute for the Sciences (IRIS). All of the students have worked in Professor Sean Murray’s research laboratory in the biology department at California State University Northridge.  Collaborators on the project include Chemistry Professor Jimmy Blair of Williams College and Biology Professor Math Cuajungco of California State University Fullerton.

Growth curves of normal and mutant bacteria were used to demonstrate that the magic spot is critical for surviving fatty acid starvation.  Relative levels of cell cycle regulator proteins were determined, as well as the degree to which individual genes were turned on or off in response to starvation.  A fluorescence-activated cell sorter was used to monitor the number of chromosomes per cell.  This research provides a model for how organisms survive starvation – by avoiding unnecessary activities such as DNA replication or increasing cell size.