Quantitative measurements of horizontal DNA transfer are critical if one wishes to address questions relating to ecology, evolution and the safe use of recombinant bacteria. Traditionally, the efficiency of a conjugation system has been described by its transfer frequency. However, transfer frequencies can be determined in many ways and may be sensitive to physical, chemical and biological conditions. In this study the authors have used the mechanistic similarity between bacterial conjugation and simple enzyme catalysis in order to calculate the maximal conjugation rate ( ) and the recipient concentration ( ) at which the conjugation rate is half its maximal value, for two different conjugation systems: the F plasmid from and plasmid pCF10 from The results are compared with the data obtained from the aggregation-mediated conjugation system encoded on pXO16 from The conjugation systems analysed are fundamentally different; however, they have some characteristics in common: they are able to sustain conjugative transfer in liquid medium and the transfer efficiencies are very high. Conjugation encoded by the F plasmid in involves the formation of small aggregates (2-20 cells), established by sex pili, and the plasmid's maximal conjugation rate was estimated to be approximately 0·15 transconjugants per donor per minute. Pheromone-induced conjugation in , which involves the formation of large aggregates, was found to proceed at a maximal conjugation rate of 0·29 transconjugants per donor per minute. Also, the value differed significantly between these conjugation systems; this may reflect the inherent differences in mating pair formation and transfer mechanisms. In these conjugation systems, the donors underwent a ‘recovery period’ between rounds of conjugative transfer and newly formed transconjugants required a period of about 40-80 min to mature into proficient donors.

Keyword(s): conjugation , F plasmid , pCF10 , pheromone and pX016

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