The int gene is essential for 89K excision A common feature of PAIs is the presence of a sitespecific recombinase that is presumably responsible for the integration of the PAI into the bacterial chromosome (Mantri and Williams, 2004) . The 89K int gene, located near the left end of the island (attL) at a distance of 63 bp, encodes a 403-amino-acid (aa) integrase belonging to the tyrosine recombinase family, which catalyses site-specific recombination between a chromosomal site (attB) and a similar or identical sequence (attP) found on mobile genetic elements including bacteriophages, insertion sequences and transposons (Esposito and Scocca, 1997; Nunes-Duby et al., 1998) . Functional studies of related members of this family give good reason to assume that 89K excision is mediated by the 89K-borne integrase. To determine if this is the case, we generated an isogenic int knockout mutant of strain 05ZYH33 (creating 05ZYH33Dint) and examined 89K excision in this mutant. PCR products were generated using primer pairs P1/P5 (primer P5 is located upstream of xis, as the int deletion removes the priming site for P2) and P3/P4. However, negative results were obtained with primer pairs P3/P5 and P1/P4 (Fig. 3B ). qPCR analysis of the Dint mutant also demonstrated that 89K excision was eliminated, with attP not being detected. Trans-complementation of int restored the excision capacity of 89K to a level comparable to that of the parental strain, with attP being present in~0.025% of cells. These results suggest that deletion of int in strain 05ZYH33 abolished the ability of 89K to excise from the host chromosome. Thus, Int is essential for 89K excision and circularization by mediating site-specific recombination between the attL and attR sites.