Replication of thermosensitive Rts1 plasmid deoxyribonucleic acid at the nonpermissive temperature

Replication of the thermosensitive drug resistance factor Rts1 was studied at the nonpermissive temperature (42 degrees C). It was concluded from the following observations that replication of this plasmid takes place at 42 degrees C without involving the covalently closed circular (CCC) form of deoxyribonucleic acid (DNA). (i) DNA-DNA- reassociation kinetics studies with purified Rts1 DNA showed that Rts1 DNA increased several-fold during cell growth at 42 degrees C while very little, if any, CCC DNA was synthesized. (ii) When Escherichia coli 20S0(Rts1) was labeled with [3H]thymidine at 42 degrees C, a significant amount of radioactive DNA hybridizable to Rts1 DNA was formed. This DNA was found in a fraction where DNA other than CCC DNA was expected in alkaline sucrose density gradient centrifugation analysis. When E. coli 20S0(Rts1) was labeled at 32 degrees C, the labeled CCC DNA did not disappear during a chase period at 42 degrees C. This indicates that preformed CCC DNA does not participate in replication at the nonpermissive temperature. These results are consistent with the hypothesis that there are two modes of replication of Rts1 DNA, one involving a CCC molecule and the other not involving this form, and that only the latter mode takes place at the nonpermissive temperature.