, 2010) and largely determined

by indirect readout of a sequence-directed DNA bend PI3K cancer occurring at an A-tract located between both subsites (O. Porrúa & F. Govantes, unpublished data). The role of ABS-3 as a repressor element and the involvement of a spontaneous DNA bend in recognition are new features of the ‘sliding dimer’ model that are likely to occur in other LTTR-activated promoters. In addition to sensing cyanuric acid, AtzR activates atzDEF transcription during nitrogen-limited growth in the absence of an inducer. Genetic evidence indicates that GlnK interacts directly with AtzR under nitrogen limitation and stimulates its activity. The nature of this interaction is currently unknown. The fact that it occurs only under nitrogen limitation indicates that the uridylylated form of GlnK is likely the physiologically

relevant form for this regulation. However, by constitutively producing a nonuridylylatable mutant of GlnK, it was shown that the nonuridylylated form partly retains the ability to stimulate check details AtzR activity (García-González et al., 2009). Activation in response to nitrogen limitation is strictly dependent on AtzR interaction with the ABS-1 and ABS-2 subsites. However, the mechanism of activation appears to be different from the ‘sliding dimer’ model: rather than causing a stable rearrangement of the AtzR–DNA complex to the activation-proficient conformation, nitrogen limitation elicits transient shifts between the active and the inactive forms (Porrúa et al., 2010) (Fig. 4d). The difference between both mechanisms is evidenced in vivo by the phenotypes of the single subsite mutants: in the presence of cyanuric acid, atzDEF expression was not affected by inactivation of ABS-3 and only moderately diminished by mutations at ABS-1 and ABS-2, indicative

of a rigid architecture in which the protein is poised in the active conformation, interaction with ABS-3 is negated and a high affinity for ABS-1 and ABS-2 is not critical (Fig. 4c). In contrast, mutations at all three ABS subsites displayed strong phenotypes on activation in response PRKACG to nitrogen limitation alone, strongly suggesting that AtzR is not committed to a rigid architecture and likely wobbles between different conformations as a function of its relative affinity for each subsite (Porrúa et al., 2010) (Fig. 4d). This dual activation mechanism has not yet been described for any other protein in the LTTR family. Since its isolation in 1995 (Mandelbaum et al., 1995), Pseudomonas sp. strain ADP has become the best-characterized organism capable of mineralizing the widely used herbicide atrazine. The atrazine-degradative pathway of Pseudomonas sp. strain ADP has been the focus of intense biochemical and genetic characterization, including the landmark sequencing of the intriguing 108-kbp pADP-1 plasmid.