Double crossover events disrupting C225 pnpA were seen for strains UNE61, UNE64, 1493 and 2483, while a single crossover event disrupted pnpA in strain 819. No tetracycline-resistant colonies were obtained from repeated transformation experiments with pCF5 using strains A198 and C305, confirming previous results that these strains are not naturally competent (Kennan et al., 1998). The phosphorylytic activity of PNPase was measured in the benign and virulent parent strains and in the pnpA mutants (Fig. 2). PNPase activity in the virulent strain A198 was significantly lower than that in the benign strain C305, consistent with the hypothesis

that PNPase acts as a virulence repressor in D. nodosus. The mean this website PNPase activity in the three virulent strains was 25% lower than that in the four benign strains (P<0.05). With the exception of pnpA mutant 2483D3, all of the mutants with the C-terminal deletion in PNPase had significantly reduced PNPase activity compared with the parent strain (Fig. 2). However, this deletion reduced PNPase activity by only 20–50%. This modest reduction in PNPase activity is consistent with similar results from E. coli, where inactivation of the KH and S1 RNA-binding domains also resulted in a modest reduction in PNPase activity (Stickney et al., 2005; Briani et al., 2007).

By contrast, deletion of the S1 domain of PNPase in Salmonella abolished phosphorylytic activity (Clements et al., 2002). The proteases secreted by virulent D. nodosus strains are, in general, more thermostable than the

proteases secreted by benign strains (Depiazzi & Richards, 1979). After heat treatment, the mean remaining protease activity for the four benign strains was significantly lower than that for the three virulent strains (Table 2), as expected. Deletion of the C-terminus of PNPase did not HA-1077 chemical structure alter the thermostability of secreted proteases from the four benign strains, or from the virulent strain UNE61, suggesting that PNPase does not act as a repressor of thermostable protease production. However, the PNPase deletion resulted in a significant reduction in protease thermostability in the virulent strain UNE64. This result is discussed in the next section. The twitching motility of the benign and virulent parent strains and pnpA mutants was measured by determining the colony diameter after growth on TAS agar plates (Fig. 3a and c). The mean colony diameter for the four benign strains, 1.21 cm, was significantly lower than the mean colony diameter for the three virulent strains, 2.66 cm, as expected, since virulent strains have been reported previously to have greater twitching motility (Depiazzi & Richards, 1985).