EFFECTS OF SALT AND TEMPERATURE ON PLASMID TOPOLOGY IN THE HALOPHILICARCHAEON HALOFERAX-VOLCANII

We report here the effect of environmental parameters, salinity, tempe rature, and an intercalating drug on plasmid topology in the halophili c archaeon Haloferax volcanii. We first studied the topological state of the plasmid pHV11 in media of different salt compositions and conce ntrations. The superhelical density of plasmid PHV11 varies in a way t hat depends on the kind of salt and on the concentrations of individua l salts. With respect to growth temperature, the plasmid linking numbe r increased at higher temperature in a linear way, contrary to what ha s been reported for Escherichia coli, in which the plasmid linking num ber decreased at higher temperature. These results suggest that some o f the mechanisms that control DNA supercoiling in halophilic Archaea m ay be different from those described for E. coil. However, homeostatic control of DNA supercoiling seems to occur in haloarchaea, as in Bact eria, since we found that relaxation of DNA by chloroquine triggers an increase in negative supercoiling.