This model, in the context of the experiments carried out in this study, is displayed in Figure 8. SiaR by itself functions as a repressor of both the nan and siaPT operons (Figure 8A). When cAMP levels are elevated, the CRP-cAMP complex can bind to its operator and partially activate expression of the transport operon, but not the catabolic operon (Figure 8B). When both GlcN-6P and CRP-cAMP are present, an activating complex is formed with SiaR that induces expression

of the two adjacent operons (Figure 8C). When the helical phase of the two operators is altered, SiaR can only regulate the nan operon while CRP can only regulate the siaPT operon (Figure 8D). Interaction between Cyclopamine mw SiaR and CRP is necessary for regulation. Figure 8 Model of SiaR and CRP regulation of the nan and siaPT operons. A. In the absence of sialic acid and cAMP, SiaR is bound to its operator and expression of the nan and siaPT operons is repressed. B. When cAMP is present, CRP binds to its operator and is able to activate the siaPT operon, but not the nan operon. C. When both GlcN-6P and cAMP are present, SiaR and CRP are active and interact to form a complex that activates both the nan and siaPT operons. D. In helical phasing experiments, insertion of one half-turn in between the SiaR and CRP operators prevents the regulators from interacting and thus maximal

activation of the nan operon is not achieved. The interaction of CRP with another DAPT transcriptional regulator is not an unusual phenomenon, however the regulation click here of the adjacent nan and siaPT operons by CRP and SiaR appears mafosfamide to operate via a novel regulatory mechanism. What makes this regulatory region unique is that it appears that the two operons are regulated by one set of operators. Other examples of divergent operons regulated by CRP and additional regulators operate by distinctly different

mechanisms. The most common mechanism is the formation of a repression loop. An example of this is in the glp regulon of E. coli [19]. As with the siaPT operon of NTHi, only one of the divergent glp operons is induced by CRP [19]. The difference between these two systems is that the repressor GlpR binds to four operators in the intergenic region and forms a repression loop [19]. The two divergent operons of the L-rhamnose catabolic regulon of E. coli utilize yet another mechanism. In addition to having multiple CRP binding sites, the two rha operons are regulated by separate transcriptional regulators, RhaR and RhaS [20]. RhaR and CRP interact to regulate the rhaSR operon while RhaS and CRP interact to regulate the rhaBAD operon [20, 21]. SiaR shares functional similarity to NagC, the regulator of N-acetylglucosamine catabolism in E. coli. Like SiaR, NagC regulates the expression of nagA and nagB, as well as a number of additional genes.