5918 Nucleic Acids Research, 2006, Vol. 34, No. 20 conventional antimicrobials. Therefore, we can speculate that cell accumulation and retention of PNA provides the potent killing activity observed in the c.f.u. assay, whereas rifampicin and ampicillin are rapidly removed from cells after plating. Anti Sok PNAs induce the characteristic ‘ghost cell’ morphology To conﬁrm that the cell killing observed was due to Hok synthesis, we studied cell morphology by phase contrast microscopy (Figure 3). Dead cells with highly condensed material in the poles and a transparent centre ‘ghost cells’ were observed in cultures of hok/sok-carrying cells treated with either anti Sok PNA or rifampicin. Again, for compari- son, the global transcription inhibitor, rifampicin was included. Rifampicin treatment of hok/sok-carrying cells leads to a rapid RNase E-mediated degradation of Sok- RNA (31). Since hok mRNA is much more stable than Sok-RNA, the decay of Sok-RNA confers derepression of hok mRNA translation, and cell killing follows. As expected, rifampicin treatment induced ghost cell morphol- ogy (Figure 3). As negative controls, we included hok/sok- deﬁcient cells and an unrelated PNA. In all cases, the samples Figure 2. Effects of anti Sok PNAs on the growth of E. coli carrying hok/sok system. (A) Turbidity in an untreated culture (open circle) and cultures including anti Sok PNAs added at 8 mM (open triangle) and 10 mM (closed square). (B) C.f.u. in an untreated culture (closed circle) and cultures containing anti Sok PNA 5 mM (closed triangle) and 10 mM (closed inverted triangle) and rifampicin at 100 mM ¼ 122 mg/ml (closed square) and 200 mM ¼ 244 mg/ml (closed diamond). Figure 3. Effect of anti Sok PNAs on cell morphology of E.coli carrying hok/ sok. PNAs and rifampicin were added to growing cells carrying control or hok/sok-containing plasmids.