Migration of Polyphosphate Granules in Agrobacterium tumefaciens

Abstract

Agrobacterium tumefaciens has two polyphosphate (polyP) kinases, one of which (PPK1_AT) is responsible for the formation of polyP granules, while the other (PPK2_AT) is used for replenishing the NTP pools by using polyP as a phosphate donor to phosphorylate nucleoside diphosphates. Fusions of eYFP with PPK2_AT or of the polyP granule-associated phosin PptA from Ralstonia eutropha always co-localized with polyP granules in A. tumefaciens and allowed the tracking of polyP granules in time-lapse microscopy experiments without the necessity to label the cells with the toxic dye DAPI. Fusions of PPK1_AT with mCherry formed fluorescent signals often attached to, but not completely co-localizing with, polyP granules in wild-type cells. Time-lapse microscopy revealed that polyP granules in about one-third of a cell population migrated from the old pole to the new cell pole shortly before or during cell division. Many cells de novo formed a second (nonmigrating) polyP granule at the opposite cell pole before cell division was completed, resulting in two daughter cells each having a polyP granule at the old pole after septum formation. Migration of polyP granules was disordered in mitomycin C-treated or in PopZ-depleted cells, suggesting that polyP granules can associate with DNA or with other molecules that are segregated during the cell cycle.