PLASMID DNA IN A GROUNDWATER AQUIFER MICROCOSM-ADSORPTION, DNAASE RESISTANCE AND NATURAL GENETIC-TRANSFORMATION OF BACILLUS-SUBTILIS

Prokaryotes can exchange chromosomal and plasmid genes via extracellul ar DNA in a process termed genetic transformation. This process has be en observed in the test tube for several bacterial species living in t he environment but it is not clear whether transformation occurs in na tural bacterial habitats. A major constituent of terrestrial environme nts are solid particles such as quartz, silt and clay, which have cons iderable surface areas and which make up the solid-liquid interfaces o f the habitat. In previous experiments the adsorption of DNA to chemic ally purified quartz and clay minerals was shown and the partial prote ction of adsorbed DNA against DNAase 1. In a microcosm consisting of n atural groundwater aquifer material (GWA) sampled directly from the en vironment and groundwater (GW) both linear duplex and supercoiled plas mid DNA molecules bound rapidly and quantitatively to the minerals. Th e divalent cations required to form the association were those present in the GWA/GW microcosm. The association was stable to extended eluti on over one week at 23-degrees-C. Upon adsorption, the DNA became high ly resistant against enzymatic degradation. About 1000 times higher DN Aase I concentrations were needed to degrade bound DNA to the same ext ent as DNA dissolved in GW. Furthermore, chromosomal and plasmid DNA b ound on GWA transformed competent cells of Bacillus subtilis. However, in contrast to DNA in solution, on GWA the chromosomal DNA was more a ctive in transformation than the plasmid DNA. The studies also reveale d that in the transformation of B. subtilis Mg2+ can be replaced by Na +, K+ or NH4+. The observations suggest that in soil and sediment envi ronments, mineral material with inorganic precipitates and organic mat ter can harbour extracellular DNA leaving it available for genetic tra nsformation.