, 1990). The atzDEF operon is transcribed divergently from a

single σ70-dependent promoter, PatzDEF, showing poor conservation at the −35 motif, a feature shared by other positively regulated promoters. The interaction of AtzR with the divergent atzR-atzDEF promoter region has been characterized in detail. AtzR binds to five consecutive major grooves overlapping the −24 motif of the PatzR promoter and the −35 motif of the PatzDEF promoter (Porrúa et al., 2010). This five-subsite structure fits well the general binding pattern described for several tetrameric LTTRs (Toledano et al., 1994; Hryniewicz & Kredich, Apoptosis inhibitor 1995; Wang & Winans, 1995). Accordingly, the two PatzDEF-distal subsites are enclosed within a G-C-rich 7-bp heptameric palindrome centered at position −65 from the atzDEF transcriptional start, bearing the conserved T-N11-A motif, conforming http://www.selleckchem.com/products/gsk-j4-hcl.html to a strong recognition element designated the repressor-binding site (RBS) (Porrúa et al., 2007). The additional three subsites conform to a weaker binding

element, designated the activator-binding site (ABS). While keeping two subunits tightly bound to the RBS, the two additional subunits can switch between two conformations: an extended conformation that interacts with the central and PatzDEF-proximal subsites and a compact conformation that interacts with the PatzDEF-distal and central subsites (Porrúa et al., 2010). This conformational change is of paramount importance to the activation mechanism and is discussed below. Two additional features

of the divergent PatzR-atzDEF promoter region are worth mentioning. First, there is a conspicuous absence of a binding site for NtrC, the activator of the PatzR promoter. Second, there is the presence of an as yet uncharacterized cis-acting element that influences atzDEF expression: serial Oxalosuccinic acid deletion analysis revealed that the removal of sequences between the atzR transcriptional start and the AtzR-binding site resulted in an ∼10-fold decrease in atzDEF expression under all conditions, whereas the general regulatory pattern is largely unaffected (Fig. 3). The nature and function of this overimposed regulation and the trans-acting factors that may be involved are currently unknown (Porrúa et al., 2007; O. Porrúa & F. Govantes, unpublished data). The regulatory gene atzR is transcribed from the single σ54-dependent PatzR promoter, which is activated by the enhancer-binding protein (EBP) NtrC in response to nitrogen limitation. However, the atzR-atzDEF promoter region does not contain an upstream activation sequence (UAS) for NtrC binding, and a sequence-specific interaction with the promoter region is not required for PatzR activation (Porrúa et al., 2009).