Abstract
ABSTRACTDespite being perceived as relatively simple organisms, many bacteria exhibit an impressive degree of subcellular organization. In Caulobacter crescentus, the evolutionarily conserved polar organizing protein PopZ facilitates cytoplasmic organization by recruiting chromosome centromeres and regulatory proteins to the cell poles. Here, we characterize the localization and function of PopZ in Agrobacterium tumefaciens, a genetically related species with distinct anatomy. In this species, we find that PopZ molecules are relocated from the old pole to the new pole in the minutes following cell division. PopZ is not required for the localization of the histidine kinases DivJ and PdhS1, which become localized to the old pole after PopZ relocation is complete. The histidine kinase PdhS2 is temporally and spatially related to PopZ in that it localizes to transitional poles just before they begin to shed PopZ and disappears from the old pole after PopZ relocalization. At the new pole, PopZ is required for tethering the centromere of at least one of multiple replicons (chromosome I), and the loss of popZ results in a severe chromosome segregation defect, aberrant cell division, and cell mortality. After cell division, the daughter that inherits polar PopZ is shorter in length and delayed in chromosome I segregation compared to its sibling. In this cell type, PopZ completes polar relocation well before the onset of chromosome segregation. While A. tumefaciens PopZ resembles its C. crescentus homolog in chromosome tethering activity, other aspects of its localization and function indicate distinct properties related to differences in cell organization.IMPORTANCE Members of the Alphaproteobacteria exhibit a wide range of phenotypic diversity despite sharing many conserved genes. In recent years, the extent to which this diversity is reflected at the level of subcellular organization has become increasingly apparent. However, which factors control such organization and how they have changed to suit different body plans are poorly understood. This study focuses on PopZ, which is essential for many aspects of polar organization in Caulobacter crescentus, but its role in other species is unclear. We explore the similarities and differences in PopZ functions between Agrobacterium tumefaciens and Caulobacter crescentus and conclude that PopZ lies at a point of diversification in the mechanisms that control cytoplasmic organization and cell cycle regulation in Alphaproteobacteria.
Highlights
Despite being perceived as relatively simple organisms, many bacteria exhibit an impressive degree of subcellular organization
Whereas wild-type A. tumefaciens cells elongate by the extension of cell wall at budding sites that form at the new pole after cell division [22], ΔpopZ cells displayed a range of aberrant morphologies that suggest defects in budding and/or cell division, including branched Y-forms, lumpy side walls, and aberrant cell lengths (Fig. 1A and B)
As reported for plasmid-based expression of PopZ-green fluorescent protein (GFP) [7], we found that mChy-PopZ is localized to a single cell pole, and that during cell division it undergoes a dramatic redistribution from the old pole to the new pole formed by the division plane (Fig. 2A, arrowheads; Movie S2)
Summary
Despite being perceived as relatively simple organisms, many bacteria exhibit an impressive degree of subcellular organization. Decades of intensive research on C. crescentus has revealed the poles to be highly complex regions that include many different regulatory proteins, including histidine kinases, response regulators, transcription factors, proteases, protease adaptors, and others that provide a wide range of influences on cellular control [3]. Many of these proteins are conserved in Alphaproteobacteria [4], and there is good evidence that at least some aspects of their function are conserved. In most of these cases, specific knowledge of protein function is limited to the C. crescentus model, and the relatively low number of known polar proteins in other alphaproteobacteria makes it difficult to gain an understanding of the connectivity of polar networks in these species
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