1 57 8366 4.31   lexA-gfp (pSC200) 1.48 57 5089 6.39 8.31 umuDC-gfp (pSC202) 0.09 31 2083 2.77   *Fluorescence threshold level is defined as the point of clear transition from basal level (large majority of cells) to high fluorescence intensity. † Designated with regard to the ATG codon. SOS genes exhibit heterogeneity Previously, single cell expression of a sulA-gfp fusion was investigated [25]. SulA is synthesized in large amounts during the SOS response and inhibits cell division by binding to FtsZ, the major Transmembrane Transporters inhibitor component of the

cell division machinery [26]. The sulA operator has a HI of 4.65 and thus binds LexA tightly. The authors found that in the absence of exogenous DNA damaging agents only approximately 0.3% of the examined

cells fully expressed sulA. As RecA is required to initiate the SOS response and LexA to repress the response, both are expressed, albeit at a low level, in the absence of DNA damage. A previous study showed a temporal program of expression of SOS genes upon DNA damage [21]. Subsequently, the response of individual cells to UV irradiation was followed by monitoring the activity of LexA repressed promoters fused to the promoterless gfp [27]. The authors found that the response is highly structured as several peaks in promoter activity were observed following DNA damaging UV irradiation. In our study we analyzed at the single cell level, the expression of the recA, lexA, and umuDC genes under physiological conditions using promoter fusions described previously TNF-alpha inhibitor [21]. Fluorescence microscopy revealed heterogeneity in the expression of all three genes. Based on fluorescence intensity, we found that the expression of recA (Figure 3) and lexA was high in a small percentage of the cells, 3.1 and 1.5%, respectively (Figure 2 and Table 3). In strains harboring the pore formers and DNase colicins transcriptional fusions to the gfp gene, heterogeneity was exhibited as a small subpopulation of highly expressing cells within the large majority of non-expressing cells. On the other hand, among the recA-gfp and lexA-gfp encoding populations, a small fraction exhibited high expression while the large majority exhibited

basal level expression. The number enough of highly fluorescent cells harboring the recA-gfp fusion and their fluorescence intensity were find more higher compared with cells hosting lexA-gfp. The HI of the recA SOS box is lower than of the lexA, predicting a higher affinity of LexA binding however, lexA harbors two SOS boxes. These results are in agreement with the higher basal level of the RecA protein compared to LexA, 7,200 versus 1,300 protein molecules per cell, respectively [28]. The higher levels of RecA protein could be explained by its roles in the SOS response, homologous recombination and its involvement in other repair mechanisms such as recombinational repair. Figure 3 Merged images of the phase contrast and fluorescence images of recA-gfp expression.