DIVERSITY OF MERCURY RESISTANCE PLASMIDS OBTAINED BY EXOGENOUS ISOLATION FROM THE BACTERIA OF SUGAR-BEET IN 3 SUCCESSIVE YEARS

Self-transmissible plasmids conferring mercury resistance were exogeno usly isolated from the bacterial populations of sugar beet roots (rhiz oplane) and leaves (phyllosphere) into a Pseudomonas putida recipient. Fifty rhizoplane plasmids and 29 phyllosphere plasmids (60-383 kb) we re purified, Numerical analysis of plasmid DNA restriction enzyme dige st patterns identified five distinct groups. Three of these plasmid gr oups were isolated from sugar beet crops grown at the same site over t hree consecutive years, demonstrating their established presence. Each group of plasmids comprised individual isolates with structural addit ions or deletions. The frequency of exogenous isolation correlated wit h factors likely to influence plant growth, bacterial activity and the physiological state of donors prior ro sampling, All plasmids investi gated conferred narrow spectrum mercury resistance with a reductase de toxification mechanism. None of the plasmids conferred resistance to a range of antibiotics, other heavy metals, or to UV, and following tra nsfer to recipient bacteria the range of carbon source utilisation was not altered. This is the first report of the persistence of Pseudomon as spp. plasmid structural types isolated over several years from a te rrestrial habitat.