To eliminate the disturbing

effect of the fusion protein (Fig. 3b), the fusion transposase producer plasmid was eliminated from five yjjY mutants and the motility of these strains was tested again. Reduced motility was observed in all cases, indicating that in (or close to) the yjjY gene, a DNA segment is located that affects motility. Because the sequence of the yjjY insertion site showed high similarity to the consensus used by the wt IS30 transposase, we tested whether the wt IS30 uses this target sequence as a hot spot. Only seven yjjY mutants were this website found to be generated by the wt IS30 out of the 222 mutants tested. These data demonstrate that the fusion transposase has a much more pronounced target preference for the yjjY hot spot (17.3%) compared with that of the wt transposase (3.2%). In this study, we have worked out and successfully applied a novel method based on IS30-mediated site-directed mutagenesis in order to produce nonflagellated S. Enteritidis mutants. The system was constructed based on the assumption that the FljA repressor component of the fusion transposase – as a DNA-binding protein – would bind to its target (the operator of fliC), and as a consequence, insertions could be concentrated with a relatively high frequency in the flagellin operon. The system constructed on the above basis worked well

and generated insertions. It turned out that the sequenced insertion sites showed pronounced similarity to the IS30 consensus sequence PRKACG of insertions (Table 1;

Olasz et al., 1998). This PD332991 indicated that the fusion transposase retained the target recognition ability of the wt IS30 transposase. Another feature of the insertions was that four target sites – called hot spots – were utilized several times. One of these hot spots was the target sequence in the fliD gene and these insertions resulted in nonmotile phenotypes. This fact could be considered as a proof of FljA-targeted transposition, because fliD is located in close proximity to the fliC operator sequence, which is the binding site of the native FljA repressor protein. These data suggested that the fusion of the FljA repressor protein modulated the target preference of the IS30 transposase and increased the frequency of integration into a new target site not preferred by the wt transposase. This result is in good agreement with earlier observations that the target preference of IS30 transposase can be modified by fusing the enzyme to unrelated DNA-binding proteins (Szabo et al., 2003 and unpublished data). Unexpectedly, another highly preferred hot spot was identified in the putative gene yjjY. Although this target site was recognized by both the wt and the fusion transposase, the frequency of the mutations generated by the IS30–FljA transposase was almost six times higher than that of the wild type (17.3% vs. 3.2%).