The Relationship Between Size, Growth Rate, and the Cell Cycle in Bacteria.

摘要

DNA replication, nucleoid segregation, and cell division must be coordinated with growth and cell size to ensure viability in all organisms. Failure to do so yields progeny with an inappropriate fraction of genetic and cytosolic material, reducing the fitness of the organism. This dissertation has sought to understand the role of cell size in two fundamental aspects of bacterial physiology: 1) How do bacteria regulate cell size in response to nutrient availability? 2) Does cell size govern progression of the cell cycle?;Growth rate and nutrient availability are primary determinants of cell size in single-celled organisms. Bacterial cells cultured in nutrient-rich circumstances are twice the size of cells grown in nutrient-poor conditions. How bacteria are able to perceive nutrient levels and amend cell size is largely undefined. In Chapter 2, I report the identification and characterization of the glucosyltransferase OpgH as a uridine diphosphate (UDP) glucose-dependent effector that coordinates Escherichia coli cell size with growth rate and nutritional status. High intracellular levels of UDP-glucose accumulate during growth in nutrient-rich conditions. In turn, UDP-glucose activates OpgH to sequester the essential division protein FtsZ, which obstructs assembly and/or maturation of the cytokinetic ring, delaying division, and increases cell size. In this way, OpgH directly gauges nutrient status and modifies cell size through the timing of division.;Cell cycle progression is regulated by cell size in all organisms. In bacteria, it has long been postulated that the achievement of a particular cell size triggers chromosomal replication. Chapter 3 of this dissertation describes a comparative study between E. coli and Bacillus subtilis examining whether cell mass determines the timing of initiation of DNA replication. Using mutants defective for cell size, my data confirms that E. coli directly ties the initiation event to cell mass. However, counter to the paradigm, the phenomenon of initiation mass is not conserved to B. subtilis, which appears to coordinate DNA replication through a cell cycle timer device.;This dissertation yields several original conclusions. First, the discovery of OpgH as a UDP-glucose-activated antagonist of FtsZ polymerization is a significant advance in the understanding of cell size control in bacteria. However, these results in tandem with the cognate pathway in B. subtilis reveal a remarkable instance of convergent evolution. Based on this, I propose that UDP-glucose and cognate UDP-glucose binding proteins are a widely conserved strategy to direct nutrient-dependent changes in cell size. In addition, the revelation that initiation of DNA replication is cell size-independent in B. subtilis adds to a growing sentiment that mechanisms controlling DNA replication are fundamentally divergent throughout bacteria.