Activity of the Xac parAB operon. The activity of the parAB operon was demonstrated by the ability of Xac parB::pPAU1 mutants to express ParB-TAP under the control of the native parAB promoter. The ParB-TAP expression vector was integrated into the parB locus of Xac by a single crossover event (B) generating the genomic structure depicted at the bottom of (B). (A) Genomic coordinates of the parAB operon. (B) Schematics of the integration of pPAU1 (carrying the parB-tap fusion) into the parB locus. Numbers above the map and around the circle indicate the sizes in base-pairs of the DNA fragments delimited; ERV, EcoRV restriction sites. (C) Southern blot analysis of Xac parB::pPAU1 mutants: genomic DNA was digested with EcoRV and subsequently probed with a DIG-labeled parB. The sizes of the DNA fragments detected correspond to those illustrated in (B). Lanes 1 and 2, Xac parB::pPAU1 mutants 1 and 2, respectively; lane 3, wild-type Xac. (D) Western blot was used to detect ParB-TAP (54 kDa) in the protein extracts of Xac parB::pPAU1 and Xac amy::pPAU1 mutants. Lanes: 1–2, Xac amy::pPAU1 mutant 1; 3–4, Xac amy::pPAU1 mutant 2; 5, Xac parB::pPAU1 mutant 1; 6, Xac parB::pPAU1 mutant 2; 7, Xac wild type (negative control); 8, Xac amy::pHF5Ca (expresses only the TAP tag of 21 kDa). The inductor xylose was added as indicated.