Plasmid‐Like Properties of the Four Virulence‐Associated Factors of Yersinia pestis

The virulence-associated factors of Yersinia pestis, which determine the abilities to produce pesticin I (Pst+), capsular fraction I antigen (Fra+), V and W antigen complex (Vwa+) and a cell-surface component for adsorption of exogenous pigments (Pgm+), were independently eliminated by cultivation of the cells in the presence of acridine orange, ethidium bromide or sodium dodecyl sulfate at a subinhibitory concentration. A virulent Y. pestis strain, Yreka, harbored at least five extrachromosomal DNA molecules of different sizes. In these molecules, a novel 13-megadalton DNA which was cured concomitantly with the elimination of the Fra factor was found, in addition to the known species of 7 and 44 megadaltons which were lost with the conversions to Pst- and Vwa-, respectively. Although the conversion to Pgm- could not be correlated with the lack of any proper extra-chromosomal DNA, the factor was transmitted to Pgm- cells with the aid of self-conjugative RP4 plasmid. The cells acquiring the Pgm factor regained virulence for mice.