The transcriptional activity of SdiA is affected not only by quorum signaling but also by other environmental factors, such as oxidation [10]. Overexpression of SdiA speeds up cell division [2] and causes apparently concomitant morphological changes; in both exponential and stationary phases, cells appear rounder and shorter [11]. Overexpression of SdiA also leads to resistance to the DNA-damaging agent mitomycin C as well as to other drugs [9, 11], while an sdiA null mutant strain was not hypersensitive to mitomycin C [11] but was more sensitive to flouroquinolones [12]. In addition, SdiA also affects the expression of a number of genes, including a decrease in expression of several motility [3] and chemotaxis [9] genes, a pleiotropic effect over the expression of genes involved in cell division, drug sensitivity, DNA replication and repair, macromolecular metabolism [9], genes involved in the glutamate-dependent acid resistance systems (AR-2) [3], and an increase in the expression of the AcrAB multidrug efflux pump proteins [12]. Strains overexpressing SdiA had a chain-type cell morphology [13]. On the other hand, SdiA is highly homologous to quorum-sensing transcription factors that belong to the LuxR family [14, 15, 16, 17]. The molecule inducer is represented by different N-acyl-L-homoserine lactones (AHLs), which are signaling molecules involved in communication between bacteria (quorum-sensing systems) [3, 7, 10, 15, 18, 19, 20, 21, 22]. AHLs might control the transcriptional activity of SdiA by enhancing its stability rather than by directly affecting its DNA-binding affinity [10]. Lee et al. showed that the transcription which SdiA mediates is controlled by indole, but whether this effect is indirect or direct is not known [23, 24]. However, it was experimentally shown that SdiA does not respond to indole, although it can inhibit SdiA activity [25]. On the other hand Dyszel et al. showed that the SdiA protein is partially dependent upon a diffusible molecule [N-(3-oxo-hexanoyl)-L-homoserine lactone] [3, 23, 24]. SdiA (EHEC) is regulated by an endogenous ligand, 1-octanoyl-rac-glycerol (OCL), which functions as a chemical chaperone placeholder, stabilizing SdiA and allowing for basal activity [26]. Accordingly, this family protein is composed of two domains: a conserved C-terminal domain which contains the DNA-binding region [16] and the N-terminal domain, which is involved in effector binding [17]. The crystal structure of intact SdiA has been determined [10]. A putative inactive conformation of SdiA was obtained using Markov modeling and dynamics simulations [27]. Review: CITS: [15130116]| Microarray analysis: CITS: [11244066]|