Dokument

Summary:
Bacteria are phylogenetically diverse and have evolved a variety of different cell shapes, life styles, and reproduction strategies. In the past decades, research has focused mainly on a few model organisms, such as Escherichia coli, Bacillus subtilis and Caulobacter crescentus that all divide by symmetric or asymmetric binary fission and are rod-shaped. Thus, the mechanisms underlying alternative propagation modes such as multiple offspring formation or budding are largely unknown. To further our knowledge in this field, we set out to study the dimorphic α-proteobacterium Hyphomonas neptunium as a representative of the stalked budding bacteria. In this work, we investigated the highly asymmetric cell division of H. neptunium, which occurs at the junction between the stalk and the bud, giving rise to two morphologically and physiologically different cell types. In the majority of bacteria, cell division is mediated by a multiprotein complex called the divisome. In H. neptunium, most of the known divisome components are conserved, and a comprehensive localization study confirmed that these proteins localize dynamically to the division site. Remarkably, for the central component FtsZ and the DNA translocase FtsK, we could observe unusual localization patterns, with both of them forming complexes at positions other than the final division site. FtsZ localized at both ends of the stalk. However, only the complex at the division site developed into a mature divisome by the recruitment of other division proteins. FtsK localized in an irregular pattern within the stalk structure, indicating that its function may go beyond mediating the last stages of chromosome segregation. Analysis of an ATPase-deficient FtsK variant and a strain with reduced FtsK expression indeed pointed towards an involvement of FtsK in DNA transport through the stalk in H. neptunium. Furthermore, we demonstrated that unlike in the closely related species, the conserved ATPaseMipZ is not a critical regulator of Z-ring positioning in H. neptunium, in contrast to its close relative C. crescentus. Since homologs of other regulatory systems are absent, a novel, yet unidentified, mechanism might position the division site in H. neptunium. Taken together, we could show that although cell division proteins are conserved among species, their spatiotemporal regulation and specific role can differ to fit the needs of distinct life styles.

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