- Cells contain pREP4 plasmid encoding lac repressor
- Lac repressor in trans ensures tightly-regulated expression
Cells in ther two E. coli Host Strains M15[pREP4] and SG13009[pREP4] carry the repressor plasmid pREP4, which constitutively expresses lac repressor at high levels, enabling trans repression of protein expression prior to IPTG induction. The pREP4 plasmid is maintained by kanamycin selection.