Studies of temperature-sensitive transfer and maintenance of H incompatibility group plasmids

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 degrees C could not facilitate transfer at 37 degrees C. Comparison of transfer characteristics of a non-thermosensitive H plasmid R831b and thermosensitive H plasmids from Salmonella from Ontario during a 2 h mating period showed that the thermosensitive phenotype inhibited transfer by about 200-fold at 37 degrees C and by 10-fold at 42 degrees C. At temperatures between 15 and 30 degrees C, 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 geographphical sources. Plasmids from Salmonella spp. isolated in Ontario did not possess this phenotype, whereas plasmids from Serratia marcescens 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 agarose gel electrophoresis. All were large, with molecular weights often exceeding 140 X 10(6). In contrast, the thermostable H plasmid R831b had a molecular weight on only 49 X 10(6).