EFFECTS OF AIR-DRYING AND WETTING CYCLES ON THE TRANSFORMING ABILITY OF DNA-BOUND ON CLAY-MINERALS

Chromosomal DNA from Bacillus subtilis and plasmid pHV14 bound on mont morillonite (M) and kaolinite (K) homoionic to Ca were subjected to re peated cycles of air-drying and wetting. After each cycle the ability of the bound DNA to transform competent cells was evaluated. Chromosom al DNA bound on clay retained its transforming efficiency after three to four air-drying and wetting cycles, whereas plasmid DNA lost the tr ansforming ability after one to two cycles. This loss was neither due to the desorption of DNA from the complexes nor to the negative effect s on DNA bound on clay of the acidic pH, that develops in the water fi lm surrounding the clay surface during the air-drying. The clay-DNA co mplexes required 6 h of wetting to regain their transforming ability. Free DNA retained its transforming ability for longer than bound DNA a nd did not require 6 h of wetting to regain its transforming ability. These results indicate that the different behaviour of DNA--cIay compl exes after air-drying is related to the molecular form of the DNA and to a conformational change in the DNA molecule bound on clay. (C) 1997 Elsevier Science Ltd.