Cleavage of the relBEF mRNA in vivo . The same RNA samples that were analyzed by northern blotting (Figure 1) were subjected to primer extension analysis shown in (Additional file 1: Figure S4). Detected 5′ ends, localization of the extension primers and hybridization probes are mapped on to the relBEF operon. Dotted lines mark cleavage sites that occur in response to several over-produced toxins. The gray bar indicates the region where detection of the cleavage sites in the relBEF mRNA was impossible owing to the plasmidal relE mRNA transcribed from pVK11.