SUMMARY: The mechanism of temperature-sensitive transfer was studied for plasmids of the H incompatibility group. Transfer depended on the temperature of the mating mixture but the growth temperature of the donor was also important, and donor cells previously grown at 26° could not facilitate transfer at 37°. Comparison of transfer characteristics of a non-thermosensitive H plasmid R831b and thermosensitive H plasmids from from Ontario during a 2 h mating period showed that the thermosensitive phenotype inhibited transfer by about 200-fold at 37 ° and by 10-fold at 42 °. At temperatures between 15 and 30 °, the thermotolerant H plasmid transferred at about the same frequency as the temperature-sensitive plasmid. Elimination of some H plasmids after growth of host cells was also observed and physical evidence of this was obtained. The characteristic of high-temperature elimination (Hte) was limited to plasmids from similar bacterial and geographical sources. Plasmids from spp. isolated in Ontario did not possess this phenotype, whereas plasmids from isolated in the United States did. Although the Tra(ts) and Hte phenotypes may both be characteristic of H plasmids, they were shown to be separate and distinct properties. The H plasmids used in this study were isolated and their molecular weights determined by agarse gel electrophoresis. All were large, with molecular weights often exceeding 140×10. In contrast, the thermostable H plasmid R831b had a molecular weight of only 49×10.


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