Acidocalcisomes and Polyphosphate Granules Are Different Subcellular Structures in Agrobacterium tumefaciens.Appl Environ Microbiol. 2020 Feb 14 [Online ahead of print]AE
Acidocalcisomes are membrane-enclosed, polyphosphate-containing acidic organelles in lower Eukaryota but have been described likewise for Agrobacterium tumefaciens (Seufferheld, M., Vieira, M., Ruiz, F. A., Rodrigues, C. O., Moreno, S., & Docampo, R. 2003. J. Biol. Chem. 278:29971-29978). This study aimed at the characterization of polyphosphate-containing acidocalcisomes in this α-proteobacterium. Unexpectedly, fluorescence microscopical investigation of A. tumefaciens cells using fluorescent dyes and localization of constructed fusions of polyphosphate kinases (PPKs) and of vacuolar H+-translocating pyrophosphatase (HppA) with enhanced yellow fluorescent protein (eYFP) suggested that acidocalcisomes and polyphosphate are different subcellular structures. Acidocalcisomes and polyphosphate granules were frequently located closely together and both near the cell poles. However, they never shared the same position. Mutant strains of A. tumefaciens with deletions of both ppk genes (Δppk1, Δppk2) were unable to form polyphosphate, but still showed cell pole-located eYFP-HppA-foci and could be stained with MitoTracker. In conclusion, A. tumefaciens forms polyP granules that are free of a surrounding membrane and thus resemble polyP granules of Ralstonia eutropha and other bacteria. The composition, contents and function of the subcellular structures that are stainable with MitoTracker and harbor eYFP-HppA remain unclear.IMPORTANCE The uptake of α-proteobacterial-like cells by ancestors of eukaryotic cells and subsequent conversion of these α-proteobacterial-like cells to mitochondria are thought to be a key step in the evolution of the first eukaryotic cells. The identification of acidocalcisomes in two α-proteobacterial species some years ago and the presence of homologs of the vacuolar proton translocating pyrophosphatase HppA, a marker protein of the acidocalcisome membrane in eukaryotes, in virtually all species within the α-proteobacteria suggests that eukaryotic acidocalcisomes might also origin from related structures in ancestors of α-proteobacterial species. Accordingly, α-proteobacterial acidocalcisomes and eukaryotic acidocalcisomes should have similar features. Since hardly any information is available on bacterial acidocalcisomes this study aimed at the characterization of organelle-like structures in α-proteobacterial cells at the example of A. tumefaciens.