AN UNUSUAL MUTATION IN REPA INCREASES THE COPY NUMBER OF A STRINGENTLY CONTROLLED PLASMID (RTS1 DERIVATIVE) BY OVER 100-FOLD

A copy number mutant of the Rts1 replicon (copy number 1-2 copies/cell ) was obtained. A one-base substitution in the sepA region results in a single amino acid change from histidine to asparagine at position 15 9. This mutation increased the plasmid copy number by up to 120-fold d epending upon the growth conditions. At 42.5 degrees C the plasmid wit h the wild type replicon was unstable while the mutated replicon was r elatively stable.