Summary: Natural plasmid transformation may be a mechanism for the horizontal transfer of non-conjugative plasmids in the marine environment, yet there are few marine model systems available for the study of this process. Using multimers of IncQ/P4 plasmids and a filter transformation assay, we have measured the effects of nutrients, salinity, temperature, as well as the development and maintenance of competence for genetic transformation in the high frequency of transformation (HFT) marine strain WJT-1C. Transformation frequency was proportional to the amount of DNA used from 0.1 to 1.0 μg DNA and was saturated at concentrations greater than 1.0 μg. Competence began in the early-exponential phase and reached a maximum at the onset of stationary phase. Once attained, competence was maintained in both spent and nutrient-free media for at least 10 d. Thus, the establishment and maintenance of competence was unique compared to previously described transformation systems. Temperatures ranging from 4 to 33 °C had no significant effect on the maximal transformation frequency of WJT-1C, but at 37 °C the transformation frequency was reduced. However, temperature did affect the rate of the transformation process. Salinities in the range 12 to 50% had no significant effect on the transformation frequency but transformation frequencies were lower at 6% and 63%. Cells were transformed equally well in nutrient-free media or rich media. The ability of this marine HFT strain to develop competence under a wide spectrum of conditions and to maintain the competent state indicates that natural plasmid transformation could occur in conditions found in tropical and subtropical estuaries.


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