The Cell Elongation Activity of Peptidoglycan Endopeptidases Antagonizes Cell Division in Escherichia Coli
CitationTruong, Thao. 2020. The Cell Elongation Activity of Peptidoglycan Endopeptidases Antagonizes Cell Division in Escherichia Coli. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractCell division in most bacteria is orchestrated by a complex of proteins organized in a ring-shaped structure at midcell known as the divisome or septal ring. In Gram-negative bacteria such as Escherichia coli, this fundamental biological process involves the coordinated constriction of three envelope layers: the inner membrane surrounding the cytoplasm, the outer membrane, and the peptidoglycan cell wall residing in the periplasmic space between the two membranes. This involves both synthesis and hydrolysis of peptidoglycan at the septum to form the new poles of the separated daughter cells. In contrast, cell elongation requires the activity of peptidoglycan endopeptidases, which cleave crosslinks to make space for insertion of new cell wall material across the length of the cell cylinder. It is not fully understood how cells regulate and coordinate these growth processes to maintain the integrity of their envelope layers.
The E. coli divisome is comprised of several dozen proteins spanning the envelope layers. To better understand the role of the nonessential divisome protein FtsP, I used transposon-insertion sequencing to identify synthetic lethal partners that render its function essential. I found evidence to support that FtsP is involved in activation of constriction. Furthermore, I found that that loss of FtsP is synthetically lethal with a defect in NlpI, a lipoprotein that regulates the levels of the cell wall endopeptidase MepS. Overexpression of MepS and another endopeptidase, MepM, is poorly tolerated in several division mutant backgrounds. Subsequent genetic analyses indicate that the endopeptidases antagonize the process of cell division in a general manner by interfering with the maturation of the divisome. Overall, these findings reveal that cells maintain an important balance between the growth processes of division and elongation. Consistent with this, I found that inactivation of native mepS suppresses several division mutants and exacerbates the growth defects of cells that are accelerated for division. In addition to demonstrating a competitive balance between division and elongation, this work suggests that the endopeptidases are capable of perturbing this balance or controlling the transition between these growth processes.
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