11The mechanisms that restrict peptidoglycan biosynthesis to the pole during elongation and re-12 direct peptidoglycan biosynthesis to mid-cell during cell division in polar-growing 13 Alphaproteobacteria are largely unknown. Here, we demonstrate that although two of the three 14 FtsZ homologs localize to mid-cell, exhibit GTPase activity and form co-polymers, only one, 15 FtsZAT, is required for cell division. We find that FtsZAT is required not only for constriction and 16 cell separation, but also for the termination of polar growth and regulation of peptidoglycan 17 synthesis at mid-cell. Depletion of FtsZ in A. tumefaciens causes a striking phenotype: cells are 18 extensively branched and accumulate growth active poles through tip splitting events. When cell 19 division is blocked at a later stage, polar growth is terminated and ectopic growth poles emerge 20 from mid-cell. Overall, this work suggests that A. tumefaciens FtsZ makes distinct contributions 21 to the regulation of polar growth and cell division.
23To initiate bacterial cell division, the tubulin-like GTPase, FtsZ, polymerizes and forms a 33 discontinuous ring-like structure at the future site of cell division [4][5][6][7][8][9][10]. The presence of FtsZ at 34 mid-cell leads to the recruitment of many proteins that function in cell division, collectively 35 called the divisome [11][12][13][14]. The divisome includes cell wall biosynthesis proteins, such as the 36 penicillin-binding protein, PBP3, and FtsW, which contribute to PG biosynthesis and remodeling 37 necessary to form new poles in daughter cells [11]. Once the divisome is fully assembled, FtsZ 38 filaments treadmill along the circumference of the mid-cell, driving the Z-ring constriction [9, 39 10]. The movement of FtsZ filaments is correlated with the movement of enzymes that function 40 in septal PG biogenesis. These finding are consistent with the notion that FtsZ not only recruits 41 enzymes that function in PG biogenesis to mid-cell but also regulates their activities to promote 42 proper cell wall biogenesis [15-17].43 44In most rod-shaped model organisms used to study cell division, a block in cell division leads to 45 the production of long, smooth filamentous cells. This phenotype suggests that assembly or 46 activation of some divisome components is necessary not only to enable the cells to divide but 47 also to stop cellular elongation. Indeed, in Escherichia coli, FtsZ (along with the Z-ring 48 stabilizing proteins FtsA, ZipA, and ZapA) has been proposed to have an early function in the 49 switch from lateral PG biogenesis to mid-cell PG biosynthesis [18]. Following maturation of the 50 divisome by recruitment of additional PG remodeling enzymes and cell division proteins, PG 51 biosynthesis is coordinated with membrane invagination, enabling cells to constrict and separate 52 [19].53 54 4Conversely to e.g. E. coli, polar growing rods in the alphaproteobacterial clade Rhizobiales 55 exhibit branched morphologies when cell division is blocked [20][21][22][23][24][...