Abstract
Polyphosphates (polyP) are ubiquitous molecules present in most organisms including bacteria, archaea and eukaryotes. Although several physiological functions have been attributed to polyP in addition to being a reservoir of phosphate, nothing is known about the possible role of polyP in archaea. A model for the participation of polyP in the tolerance to heavy metals has been proposed in bacteria. The intracellular concentrations of these ions would stimulate polyP degradation and the hydrolyzed phosphate obtained by the action of PPX could be transported out of the cell along with the metal cations. To study if such a system exists in Archaea, the presence of polyP was determined by the electron energy loss spectroscopy (EELS) procedure and quantified by using specific enzymatic methods in S. acidocaldarius, S. metallicus and S. solfataricus. All three microorganisms synthesized polyP during growth, but only S. metallicus greatly accumulated polyP granules. The differences in the capacity to accumulate polyP between these archaeons may reflect adaptive responses to their natural environment. Thus, S. metallicus was able to grow and tolerate up to 200 mM copper sulfate, with a concomitant decrease in its polyP levels with increasing copper concentrations. On the other hand, S. solfataricus could not grow in or tolerate more than 1-5 mM copper sulfate, most likely due to its low levels of polyP. Shifting S. metallicus cells to copper sulfate concentrations up to 100 mM showed a rapid increase in their PPX activity which was concomitant in time with a decrease in their polyP levels and a stimulation of phosphate efflux. Furthermore, copper in the range of 10 μM greatly stimulated PPX activity in cell-free extracts from S. metallicus. The results strongly suggest that a metal tolerance mechanism mediated through polyP is also functional in members of the Sulfolobus group.
