FREQUENCY OF HORIZONTAL GENE-TRANSFER OF A LARGE CATABOLIC PLASMID (PJP4) IN SOIL

Limited work has been done to assess the bioremediation potential of t ransfer of plasmid-borne degradative genes from introduced to indigeno us organisms in the environment. Here we demonstrate the transfer by c onjugation of the catabolic plasmid pJP4, using a model system with do nor and recipient organisms. The donor organism was Alcaligenes eutrop hus JMP134 and the recipient organism was Variovarax paradoxus isolate d from a toxic waste site. Plasmid pJP4 contains genes for mercury res istance and 2,4-dichlorophenoxyacetic (2,4-D) acid degradation. A tran sfer frequency of approximately 1/10(3) donor and recipient cells (par ent cells) was observed on solid agar media, decreasing to 1/10(5) par ent cells in sterile soil and finally 1/10(6) parent cells in 2,4-D-am ended, nonsterile soil. Presumptive transconjugants were confirmed to be resistant to Hg, to be capable of degrading 2,4-D, and to contain a plasmid of size comparable to that of pJP4. In addition, we confirmed the transfer through PCR amplifications of the tfdB gene. Although tr ansfer of pJP4 did occur at a high frequency in pure culture, the rate was significantly decreased by the introduction of abiotic (sterile s oil) and biotic (nonsterile soil) stresses. An evaluation of the data from this model system implies that the reliance on plasmid transfer f rom a donor organism as a remediative strategy has limited potential.