Out of 32 R plasmids isolated from
Proteus strains, 17 were found to be temperature sensitive with respect to inheritance in
E. coli cells. They were
fi- and classified into incompatibility group T or V. Cells carrying T group Rms273 plasmid were temperature sensitive with respect to growth and conjugal transfer in both
E. coli and
Proteus. The V group YOR-10 plasmid was stable in
Proteus even at 42 C. However, the loss frequency of YOR-10 plasmid in
E. coli reached 100% after 4 hr of incubation at 42 C, in spite of stable inheritance at 25 C. Conjugal transfer of the YOR-10 plasmid in
E. coli was also strongly inhibited at 42 C. It has been concluded that instability of V group R plasmids in
E. coli is due to their thermosensitive inheritance in the progeny cells at high temperatures.
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