Figure 5.

Repression of the SPI-1 genes by CpxR is lost in the absence of the Lon protease. Expression of the SPI-1-encoded InvF-FLAG in the WT S. Typhimurium strain and its isogenic ΔcpxA, ΔhilE, ΔcpxA ΔhilE, Δlon and ΔcpxA Δlon mutants (A), or in the WT S. Typhimurium strain and its isogenic ΔhilE and Δlon mutants carrying plasmid pCA-CpxR (C), was analyzed by Western blotting using monoclonal anti-FLAG antibodies. Whole cell lysates were prepared from samples of bacterial cultures grown for 5 h in LB medium at 37°C. As a loading control, the expression of DnaK was also determined using monoclonal anti-DnaK antibodies. Overexpression (+) of CpxR from the T5-lac promoter of plasmid pCA-CpxR was induced by adding 50 μM IPTG at the beginning of the bacterial cultures. (B) Secretion analysis of the SPI-1-encoded proteins SipA, SipB, SipC, and SipD was tested in the WT S. Typhimurium strain and its isogenic ΔcpxA, ΔhilE, ΔcpxA ΔhilE, Δlon, and ΔcpxA Δlon mutants grown for 9 h in LB medium at 37°C. FliC is a flagellar protein whose secretion is SPI-1-independent.