Figure 4 .

Deletion mutations that increase lacZ expression and allow tester growth on lactose. Many of the characterized revertants in the Pur-Lac system have acquired a deletion of the region between the purD coding sequence and the upstream end of the inserted MudJ element (mDEL). Of 22 sequenced mDEL deletions, 21 removed 106 bp (mDEL1) and 1 removed 260 bp (mDEL2). Both deletion types remove material downstream of the purD UGA mutation including an interrupted palindromic sequence element. The mDEL3 and 4 deletions arose secondarily in strains carrying mDEL1 and improved growth further by removing the polar UGA mutation.