Acidocalcisomes and Polyphosphate Granules Are Different Subcellular Structures in Agrobacterium tumefaciens.

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 -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+45 -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. [ABSTRACT FROM AUTHOR]