SepIVA has a function in activating the mycobacterial divisome, which is inhibited during DNA damage

Bacterial cell division is a tightly regulated process that is carried out by a complex of proteins called the divisome, which is assembled in a defined sequential order. Upon assembly the complex is allosterically activated, which stimulates cell wall synthesis at the division site. Bacteria inhibit division during DNA damage by blocking either divisome assembly or activation. While the regulation of cell division is known to be important during M. tuberculosis infection, little is known about mycobacterial mechanisms of divisome function and regulation in DNA damage. By using M. smegmatis as a model organism, we find here that divisome factor SepIVA is involved in septum initiation, and that it is recruited to the mid-cell by FtsQ but it is not a recruitment factor itself. We find that a sepIVA loss-of-function defect can be suppressed by overexpression of ftsW, supporting a role for SepIVA in activation of the divisome complex. When cell division is inhibited during DNA damage, we find that SepIVA is delocalized from the division site, while the septal localization of FtsZ, FtsQ and FtsW are not impacted. We also find that the interaction between FtsQ and SepIVA is inhibited during DNA damage. Our results suggest that SepIVA is a trigger factor for activation of cell division during normal growth, and show that the signaling to inhibit cell division during DNA damage involves inhibition of its interaction with FtsQ. IMPORTANCECell division is critical for bacterial cells to propagate and cause infection. Despite its importance, division is a dangerous process as it requires building and subsequently hydrolyzing new cell wall material, in a place where the chromosome typically resides. If cell division is done before chromosome segregation is completed, or if cell wall metabolism is improperly regulated, then cell death results. The divisome is a protein complex that regulates cell division - coordinating it with the status of the chromosome and ensuring that the cell wall metabolic enzymes are carefully controlled. Many of the proteins in the divisome complex are highly conserved across bacterial Phyla; however, the factor that stimulates the complex to activate and initiate cell wall synthesis is not widely conserved. Here, we study the activation of the divisome in Mycobacterium smegmatis, a model for cell physiology of Mycobacterium tuberculosis. We find evidence that SepIVA, a protein found only in Actinobacteria, is likely the factor that stimulates activation of the divisome in mycobacteria. We also show that the association of SepIVA with the divisome is blocked under DNA damage, when cell division is inhibited. These results provide a model for the regulation of cell division in mycobacteria in growth and stress, and also provide insights into how bacteria with different types of septa may regulate division differently.