ACCESSORY PROTEINS IMPOSE SITE SELECTIVITY DURING COLE1 DIMER RESOLUTION

The cer-Xer dimer resolution system of plasmid ColE1 is highly selecti ve, acting only at sites on the same molecule and in direct repeat. Re combination requires the XerCD recombinase and accessory proteins ArgR and PepA, The Escherichia coil chromosome dimer resolution site dif a nd the type II hybrid site use the same recombinase but are independen t of ArgR and PepA and show no site selectivity. This has led to the p roposal that ArgR and PepA are responsible for the imposition of const raint. We describe here the characterization of a novel class of 'cond itionally constrained' multimer resolution sites whose properties supp ort this hypothesis. In the presence of ArgR and PepA, plasmids contai ning conditionally constrained sites are monomeric, but in their absen ce, extensive multimerisation is seen, A mutant ArgR derivative (ArgR1 10), which is defective in cer-mediated dimer resolution, remains able to prevent plasmid multimerisation by a conditionally constrained sit e. This implies that the accessory factors block recombination in tran s rather than ensuring rapid multimer resolution. When the distance be tween the ArgR and XerCD binding sites in a conditionally constrained site was altered by a non-integral number of helical turns, the site b ecame unconstrained. Constraint was restored by the insertion of a ful l helical turn.