RETROMOBILIZATION OF HEAVY-METAL RESISTANCE GENES IN UNPOLLUTED AND HEAVY-METAL POLLUTED SOIL

Retromobilization of the nonconjugative (Tra(-)Mob(+)) IncQ vector, pM OL155, and the non-mobilizable (Tra(-)Mob(-)) vector, pMOL149, by mean s of the IncP plasmids RP4 and pULB113 (RP4::Mu3A), was studied in pla te matings and in soil microcosms, and compared with direct and tripar ental mobilization. Both vectors harbour the czc genes, originating fr om Alcaligenes eutrophus, which code for resistance to Co, Zn, and Cd, The donor of the czc genes was Escherichia coli which did not express these genes. The recipient, Alcaligenes eutrophus, expressed the czc genes very well. Retromobilization, direct and triparental mobilizatio n of pMOL155 was observed in sterile soil. Both the addition of nutrie nts and heavy metals significantly enhanced the number of (retro)trans conjugants. Retromobilization was also detected in nutrient amended no nsterile soil, but the presence of the autochthonous soil biot a stron gly reduced the number of retrotransconjugants and also prevented thei r increase upon application of heavy metals to the soil. Retromobiliza tion of the czc genes, cloned in pMOL149, by using pULB113 was also ob served, yet only in sterile, nutrient amended, heavy metal polluted so il.