Functional analysis of polyphosphate in Myxococcus xanthus

Abstract Myxococcus xanthus synthesizes polyphosphates (polyPs) with polyphosphate kinase 1 (Ppk1) and degrades short- and long-chain polyPs with the exopolyphosphatases, Ppx1 and Ppx2, respectively. M. xanthus also expresses polyP:AMP phosphotransferase (Pap) which generates ADP from polyPs and AMP. M. xanthus synthesized polyPs during the stationary phase; the ppk1 mutant showed no difference in growth during the exponential growth phase but died earlier than the wild-type and ppx2 mutant after the stationary phase. In addition, M. xanthus cells cultured in phosphate-starved medium, 0.07 mM H2O2-supplemented medium, or nutrient-deficient medium (CF medium) increased the intracellular polyP levels by six- to eight-fold after 6 h of incubation. However, the growth of ppk1 and ppx2 mutants in phosphate-starved medium and 0.07 mM H2O2 supplemented medium was not significantly different from that of wild-type strains, nor was there a significant difference in fruiting body formation and sporulation on CF medium. The adenylate energy charge (AEC) values of the wild-type strain and the ppk1, ppx2, and pap mutant strains at the exponential growth phase were approximately 0.80. The AEC of the wild-type strain did not change until day 3 of development, whereas the AEC values of the ppk1 and pap mutant strains decreased to 0.77 due to an increase in AMP and a decrease in ADP. Spores of ppk1 and pap mutants in the nutrient medium germinated later than those of the wild-type strain. These results suggested that polyPs produced during development may play an important role in cellular energy homeostasis by being used to convert AMP to ADP via Pap.