High concentrations of intracellular Ap4A and/or Ap5A in developing Myxococcus xanthus cells inhibit sporulation

Diadenosine polyphosphates (Ap(n)A) are thought to act as signalling molecules regulating stress responses and biofilm formation in prokaryotes. However, Ap(n)A function in Myxococcus xanthus remains unknown. Here, we investigated the role of Ap(n)A in M. xanthus, using the wild-type and Ap(n)A hydrolase (apaH) mutant strains exposed to various stress conditions. In both wild-type and apaH mutant cells cultured on starvation medium (CF agar), the levels of intracellular diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) increased several fold during the first 16 h of development and decreased gradually thereafter. The levels of Ap(4)A and Ap(5)A in the apaH mutant were about 5-and 11-fold higher than those in the wild-type strain at 16 h, respectively. Ap(n)A hydrolase activity of the wild-type strain increased 1.5-fold during the first 8 h of development, and it then gradually decreased. The apaH mutant formed spores 1-2 days after the wild-type strain did, and the yield of viable spores was 5.5% of that in the wild-type strain 5 days after inoculation onto CF agar. These results suggest the possibility that high intracellular levels of Ap(4)A and/or Ap(5)A may inhibit M. xanthus sporulation at the early stage of development and that the bacteria reduce intracellular Ap(4)A and Ap(5)A accumulation through Ap(n)A hydrolase activity.