Studies of Temperature-sensitive Transfer and Maintenance of H Incompatibility Group Plasmids

Abstract

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.degree. C could not facilitate transfer at 37.degree. C. Comparison of transfer characteristics of a nonthermosensitive H plasmid R831b and thermosensitive H plasmids from Salmonella from Ontario, Canada, during a 2 h mating period showed that the thermosensitive phenotype inhibited transfer by about 200-fold at 37.degree. C and by 10-fold at 42.degree. C. At temperatures between 15-30.degree. 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 geographical sources. Plasmids from Salmonella spp. isolated in Ontario did not possess this phenotype, whereas plasmids from Serratia marcescens isolated in the USA did. Although the Tra(ts) and Hte phenotypes may both be characteristic of H plasmids, they were separate and had distinct properties. The H plasmids used in this study were isolated and their MW determined by agarose gel electrophoresis. All were large, with MW often exceeding 140 .times. 106. In contrast, the thermostable H plasmid R831b had a MW of only 49 .times. 106.