Instability of Rts1 (Drug-Resistant Factor) Replicon: Stabilization by DNA Fragments Derived from Rts1

Rts1 is a large naturally occurring plasmid which has a kanamycin resistance gene and exhibits various temperature-sensitive phenotypes. A smaller derivative of plasmid, pOK, contains the Rts1 replicon and the kanamycin resistance gene of Rts1. This plasmid, pOK, is much more unstable than Rts1 at 42.5°C. A DNA fragment, G₃, 1590 nucleotides long from Rts1 DNA, stabilized pOK completely at 42.5°C but only in thecisconfiguration. G₃did not change the copy number of pOK. The pOK derivative containing G₃was destabilized by the presence of a compatible plasmid containing G₃. G₃has four inverted repeats, two 14-base direct repeats, and three ORFs. Smaller fragment of G₃also had a stabilization effect and these studies showed that the ORF does not play any role in stabilization.