 First, promoter regions were replaced with a more efficient hybrid promoter which combined the "E. coli" phage lambda PL (λ PL) promoter with "lac repressor" ("Lacl") and "Tet repressor" ("TetR") operator sequences.Second, to reduce the disparity between the lifetimes of the repressor proteins and the mRNAs, a carboxy terminal tag based on the ssrA-RNA sequence was added at the 3' end of each repressor gene.This tag is recognized by proteases which target the protein for degradation.