Summary: The rickettsial pathogen undergoes a variation in which virulent isolates (phase 1) become avirulent (phase 2) after repeated passage in a non-immunologically competent host. Biochemically, this variation is associated with a lipopolysaccharide modification and possibly other factors. Genetically, the regions of DNA responsible for phase variation have not been identified. We have sought to determine whether the plasmid identified in acute disease isolates, QpHl, which represents approximately 5% of the coding capacity of this organism is involved in phase variation. Plasmids from phase 1 and phase 2 variants (designated QpHl and QpH2, respectively) were compared by restriction endonuclease digestion and Southern blot hybridization to determine whether sequence changes in the phase 2 plasmid might account for changes in the virulence of phase 2 organisms compared with that of phase 1 cells. Using over 20 different restriction enzymes, no changes in DNA restriction fragment patterns were detected regardless of whether the phase change occurred during egg or tissue culture passage. The plasmid-specific mRNAs produced from metabolically active, purified cells were identical for each phase type. Using QpH1 or QpH2 DNA as a template, the mRNA produced by an extract was also identical. Finally, the proteins encoded by either plasmid in an transcription/translation reaction were identical. These data indicate that within the limits of our analysis, the plasmid DNA from phase variants is structurally and functionally the same and is therefore unlikely to be involved in phase variation.


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