Mechanism of retarded DNA degradation and prokaryotic origin of DNases in nonsterile soils

DNA released from organisms exits at considerable quantities in water, sediment and soil. Such DNA can serve as genetic information for transformable bacteria. It is not clear so far how DNA persists in an environment such as soil where DNase activity is present. The fate of linear duplex DNA (41.5 kbp) introduced into non-sterile soil microcosms was followed by monitoring the distribution of DNA to solid and liquid soil constituents and by measuring the degradation of DNA in each of the two fractions by DNases produced by microorganisms indigenous to the soils. Three non-sterile soils collected from agricultural field plots including a brown earth, a loamy soil and a podzol were examined using [H-3]-thymidine-labeled DNA. Depending on the soil type, DNA adsorption reached an equilibrium within 1 to 3 h at 23 degrees C. The capacity of the three soils to bind DNA (>13 mu g per g of soil) was sufficiently high to adsorb the total DNA of the microbial community living in them. The introduced DNA distributed between the solids and the liquid phase in soil-specific proportions. Kinetic analysis indicated that introduced DNA remaining non-adsorbed was rapidly rendered acid-soluble (within 24 h at 23 degrees C). In contrast, a large portion of the DNA that bound to the soil particulate material was of high molecular size (up to several kbp) still 5 days after introduction as determined by Southern transfer hybridization. The observations indicate that adsorption of DNA on solid soil components and the concomitant protection against DNases is the mechanism of DNA persistence in soil. Upon hydration of dry soil in the microcosms it was observed that growth of prokaryotes was associated with increased DNase activity in the interstitial soil solution. Suppression of growth by rifampicin and chloramphenicol, but not by cycloheximide, prevented the appearence of DNases. This suggests growing prokaryotes as the main producers of DNases in the soil microcosms.