075 26 9315 cna-gfp (pSC301) 0.45 26 8938 ce1a-gfp (pSC302) 0.36 26 7253 ce7a-gfp (pSC303) 0.43 26 7050 recA-gfp (pSC201) 1.69 52 5695 lexA-gfp (pSC200) 0.53 52 2823 umuDC-gfp (pSC202) 0.05 24 4004 *Fluorescence threshold level is defined as the point of clear transition from basal

level (large majority of cells) to high fluorescence intensity. Both the recA-gfp and lexA-gfp fusions were expressed in the recA defective strain RW464, albeit at a lower level compared to the wild type (Table 4, Figure 2, Figure ACP-196 3), with a small fraction of the population exhibiting high fluorescence indicating that, stochastic factors could be involved. Filamentation due to delay in cell division is evident among the less robust recA defective strain. However, expression of the investigated genes was not limited to filamented cells (Figure 3). To resolve the effect of LexA regulation at the single cell level, expression of the investigated gene fusions was also studied in strain RW542 encoding a LexA protein defective in binding to LexA boxes. Fluorescence microscopy revealed

that in the lexA defective strain all cells harboring the lexA-gfp or recA-gfp fusions, as well as the large majority (98%) of the cells harboring gfp fusions with the colicin activity genes were intensely fluorescent, indicating high level expression click here (data not shown). Simultaneous expression of the cka and SOS genes The advent of novel fluorescence markers enables analysis of simultaneous expression of two or more genes. To investigate in detail how the expression of colicin genes correlates with the expression of SOS genes, simultaneous expression of the cka and the lexA genes was followed at the single cell level in strain RW118 harboring two plasmids: pKCT10 with a cka-DsRed-Express2 fusion and the pSC101 derivative vector harboring the lexA-gfp fusion. As is evident from Figure 4, the large majority of cells that more highly expressed the lexA gene also expressed the cka gene. Nonetheless, individual

cells (approximately 0.1%) highly expressing only the cka gene could be detected suggesting, that in a very small fraction of the population the colicin K activity gene is expressed in the absence of the SOS response most probably stochastically, due to perhaps Dehydratase intracellular fluctuations of the LexA protein. Filamentation while a hallmark of SOS induction due to binding of SulA to the FtsZ proteins is also evident in cells not expressing lexA-gfp (Figure 4). Multimers of the natural cka-gfp encoding plasmid could be responsible for filamentation in the absence of SOS induction [38]. Figure 4 Fluorescence images showing simultaneous expression of the cka-DsRed-Express2 and lexA-gfp transcriptional fusions. A:. Expression of cka-DsRed-Express2 gene fusion. B: Expression of lexA-gfp gene fusion in RW118.