Figure 1.

Screening system to isolate exonuclease-defective mutants of L. lactis. RC plasmid replication may generate a σ-shaped molecule with dsDNA tail (Upper; arrow indicates direction of replication). This tail may be degraded by exonuclease, thereby restoring a monomeric circle (Left Lower). Expression of the nuc reporter gene (present on pRC1-nuc) is low and results in weak halos around colonies. If the exonuclease is inactivated, the σ form is extended and HMW accumulates, amplifying the nuc gene. Amplification leads to overexpression of Nuc, which gives rise to strong halos of DNA digestion around colonies in plate tests (Right Lower).