Chromosomal DNA from Bacillus subtilis and plasmid pHV14 bound on montmorillonite (M) and kaolinite (K) homoionic to Ca were subjected to repeated cycles of air-drying and wetting. After each cycle the ability of the bound DNA to transform competent cells was evaluated. Chromosomal DNA bound on clay retained its transforming efficiency after three to four air-drying and wetting cycles, whereas plasmid DNA lost the transforming ability after one to two cycles. This loss was neither due to the desorption of DNA from the complexes nor to the negative effects on DNA bound on clay of the acidic pH, that develops in the water film surrounding the clay surface during the air-drying. The clay-DNA complexes required 6 h of wetting to regain their transforming ability. Free DNA retained its transforming ability for longer than bound DNA and did not require 6 h of wetting to regain its transforming ability. These results indicate that the different behaviour of DNA-clay complexes after air-drying is related to the molecular form of the DNA and to a conformational change in the DNA molecule bound on clay
