PopZ and FtsZ coordinate polar growth termination and cell division in Agrobacterium tumefaciens
Understanding how bacterial cells expand their cell walls is an important question with relevance to development of antibiotics. While many studies have focused on the regulation of bacterial elongation utilizing lateral cell wall biogenesis, polar growth in bacteria is less well understood. Yet, polar growth has been observed across taxonomically diverse bacteria like Actinobacteria and the alphaproteobacterial clade Rhizobiales (Howell and Brown, 2016). Interestingly, polar-growing bacteria within Rhizobiales lack canonical scaffolding proteins for spatial and temporal regulation of peptidoglycan synthesis during elongation. Here, we dissect the role of two candidate scaffolding proteins in directing cell wall synthesis in the bacterial plant pathogen, Agrobacterium tumefaciens. Since cell wall (peptidoglycan) biosynthesis during elongation and cell division is vital for bacterial survival, we expected many key proteins involved in these processes to be essential for cell survival. Thus, we developed a depletion system for A. tumefaciens (Figureroa-Cuilan et al. 2016). We further optimized a suite of target-specific fluorescent labeling techniques which allow us to visualize morphological changes during essential cell processes (Howell, Daniel, and Brown, 2017). We use these techniques to dissect the contributions of PopZ and FtsZ to polar growth and cell division. Although PopZ is not required for polar growth, it is required for proper coordination of polar growth, chromosome segregation, and cell division. This PopZ-mediated coordination ensures that daughter cells are the proper size and contain a complete complement of genetic material (Howell et al 2017). Next, we find that FtsZ is required for both termination of polar growth and cell division. This finding suggests that FtsZ has at least two important functions in regulation of cell wall biogenesis. First, FtsZ enables cell wall biogenesis machinery to be released or inactivated from the growth pole. Second, FtsZ must recruit additional proteins to mid cell to assemble the divisome, enabling activation of cell wall biogenesis to promote septum formation and cell separation. While further research is needed to understand how growth is targeted to the pole during elongation, our work provides mechanistic insights about the coordination of polar growth termination, chromosome segregation, and cell division. We hypothesize that our findings will be applicable to other closely related polar growing Rhizobiales, including plant, animal, and human pathogens.
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