RETROTRANSFER KINETICS OF R300B BY PQKH6, A CONJUGATIVE PLASMID FROM RIVER EPILITHON

The kinetics of conjugation, retrotransfer and mobilisation were studi ed at 5-15 min intervals between strains of Pseudomonas putida using p lasmid pQKH6, isolated from river epilithon, and R300B. Transconjugant s from the direct conjugation of pQKH6 and mobilisation of R300B by pQ KH6 appeared rapidly, reaching maximum densities within 30-60 min of t he start of both filter and liquid mating experiments. However, retrot ransconjugants only appeared after a delay. This delay was short (appr ox. 45-60 min) in filter mating and much longer (2-5 h) for liquid mat ing experiments. Attempts at predicting the time course of retrotransc onjugant development from (i) numbers of transconjugants from the conj ugation and mobilisation experiments and (ii) mathematical models base d on a mass action approach, both failed to reproduce the observed del ay. It was concluded that retrotransfer did not proceed by either a on e-step mechanism occurring early in conjugation or two separate conjug ation and mobilisation steps. The clear demonstration of a delay in re trotransconjugant formation implies that a new mechanism must be sough t. The likely importance of retrotransfer in the environment is discus sed.