In all cases, killing curves were performed with two different spore preparations, and these yielded essentially similar (±20%) results. selleck products Survivors of wet heat treatment were transferred onto either minimal medium or sporulation agar plates and incubated for 24–48 h to assess the percentage of survivors that had acquired auxotrophic or asporogenous mutations as described previously (Fairhead et al., 1993). We decided to use the strong PsspB promoter
to overexpress Nfo, because PsspB has yielded high-level expression of several proteins in spores (Paidhungat & Setlow, 2001; Cabrera et al., 2003). To confirm that PsspB in our construct was indeed forespore-specific, we used this promoter to drive GFP expression, and examined sporulating cells of the PsspB-gfp strain (PERM751) by fluorescence microscopy (Fig. 1a). The results showed that in around 30% of analyzed sporangia, GFP was clearly accumulated to significant levels in developing
spores (Fig. 1a, arrows), and there was no noticeable fluorescence in the mother cell compartment of sporulating cells. The above results indicated that the PsspB we planned to use to overexpress Nfo is indeed forespore-specific. SDS-PAGE of extracts of spores of strains with or without nfo under PsspB control (Fig. 1b) showed that spores of a B. subtilis strain (PERM641) with PsspB-nfo contained a prominent band at 33 kDa, the expected molecular mass of Nfo (Salas-Pacheco et al., 2003), Z VAD FMK while this band was not prominent in extracts from spores of strains in which nfo was not controlled by PsspB (PERM450 and PS832) (Fig. 1b). These results indicate that PsspB directs forespore-specific overexpression of nfo in strain PERM641, and densitometry indicated that Nfo was overexpressed ∼50-fold in the spores of this strain (Fig. 1b, bottom). A similar level of Nfo overexpression was observed in spore extracts of the wild-type strain containing the
PsspB-nfo construct (Fig. 1b, bottom). Previous work has suggested that it is generation of AP sites in α−β−, but not wild-type spore DNA that sensitizes α−β− spores to wet heat (Setlow, 2006). With α−β− spores, only the absence of two AP endonucleases, ExoA and Nfo, decreased these spores’ resistance to wet heat 17-DMAG (Alvespimycin) HCl (Salas-Pacheco et al., 2005). Therefore, the exoA nfoα−β− genetic background was used to investigate the effects of elevated Nfo levels on spore resistance to wet heat and other treatments. As found previously (Salas-Pacheco et al., 2005), spores of the exoA nfoα−β− strain were very sensitive to wet heat (Fig. 2a and b). However, overexpression of Nfo decreased the rate of wet heat killing of nfo exoAα−β− spores significantly, and the LD90 value, the time for 90% wet heat killing at 90 °C, increased from 7.5 min for nfo exoAα−β− spores to ∼45 min for the nfo exoAα−β− spores overexpressing Nfo (Fig. 2a and b). Indeed, the wet heat resistance of the latter spores was slightly higher than that of wild-type PS832 spores (Fig.