2c). No cleavage was observed when the XerSY314F mutant was used instead of the wild-type protein (data not shown). The vector pBEA756 possesses both gram-positive thermosensitive (Ts) and ColE1 replication origins. An internal fragment of the S. suis xerS gene was generated by PCR and cloned into this vector, generating the plasmid pBEA756XerCint. This plasmid was then successfully introduced into S. suis by electroporation

as described in section ‘Growth conditions and DNA manipulations’. At the restrictive temperature (37 °C), homologous recombination events were selected for by maintaining growth in the presence of kanamycin. Bcl2 inhibitor A single crossover event between the cloned xerS gene on the plasmid and the chromosomal copy of xerS resulted in the inactivation of the xerS gene, which was confirmed by PCR and by Southern blot (data not shown). Microscopic analysis of xerS mutant cells showed a significant increase in average chain length, with most of wild-type cells being 5–10 cells long, while mutants were more

than 10 cells long; in addition, extremely long chains, containing more than 30 cells, were also observed (Fig. 3). The re-introduction of a functional xerS with pGXerCF (pGhost9) restored the wild-type phenotype (data not shown). In this report, we described the purification and inactivation of the S. suis xerS gene and its MBP-fused product. The S. suis XerS recombinase was overexpressed and purified as a maltose-binding Ribonucleotide reductase protein fusion, as previous work with XerCD recombinases has indicated that the N-terminal MBP moiety has no significant effect on Xer binding, cleavage Dabrafenib chemical structure or strand transfer activity (Blakely et al., 1997, 2000; Neilson et al., 1999; Villion & Szatmari,

2003). The difSL site was located about 50 bp before the start of the xerS coding region, as was found for most of the lactococci and streptococci (Le Bourgeois et al., 2007). In addition, XerS of S. suis displays 70% identity and 82% similarity to XerS of Lactococcus lactis (Le Bourgeois et al., 2007). Specific binding of difSL was detected at MBP-XerS concentrations of 3.43 nM and above, in the presence of a 1000-fold molar excess of poly dIdC competitor (Fig. 1a). The observation of more than one complex suggests that MBP-XerS is binding to both half-sites of difSL, which is consistent with other systems using one recombinase like Flp and Cre. Binding to the left half-site was detected, while virtually no retarded bands were visible in reactions on the right half-site (Fig. 1b,c), in agreement with results found by Nolivos et al. (2010) on the lactococcal difSL site. The faster migrating bands correspond to the binding of a single XerS monomer on the DNA, while the slower migrating forms correspond to the binding of two or more XerS protomers on the DNA. The additional retarded complexes seen with the difSL left half-site are most likely additional monomers binding to the complex via protein–protein interactions.