An unusual mutation in RepA increases the copy number of a stringently controlled plasmid (Rts1 derivative) by over one hundred fold.

A copy number mutant of the Rts1 replicon (copy number 1-2 copies/cell) was obtained. A one-base substitution in the repA region results in a single amino acid change from histidine to asparagine at position 159. This mutation increased the plasmid copy number by up to 120-fold depending upon the growth conditions. At 42.5 degrees C the plasmid with the wild type replicon was unstable while the mutated replicon was relatively stable.