The Aer2 chemoreceptor from Pseudomonas aeruginosa contains a PAS sensing domain that coordinates b-type heme and signals in response to the binding of O 2 , CO, or NO. PAS-heme structures suggest that Aer2 uniquely coordinates heme via a His residue on a 3 10 helix (H234 on E), stabilizes O 2 binding via a Trp residue (W283), and signals via both W283 and an adjacent Leu residue (L264). Ligand binding may displace L264 and reorient W283 for hydrogen bonding to the ligand. Here, we clarified the mechanisms by which Aer2-PAS binds heme, regulates ligand binding, and initiates conformational signaling. H234 coordinated heme, but additional hydrophobic residues in the heme cleft were also critical for stable heme binding. O 2 appeared to be the native Aer2 ligand (dissociation constant [K d ] of 16 M). With one exception, mutants that bound O 2 could signal, whereas many mutants that bound CO could not. W283 stabilized O 2 binding but not CO binding, and it was required for signal initiation; W283 mutants that could not stabilize O 2 were rapidly oxidized to Fe(III). W283F was the only Trp mutant that bound O 2 with wildtype affinity. The size and nature of residue 264 was important for gas binding and signaling: L264W blocked O 2 binding, L264A and L264G caused O 2 -mediated oxidation, and L264K formed a hexacoordinate heme. Our data suggest that when O 2 binds to Aer2, L264 moves concomitantly with W283 to initiate the conformational signal. The signal then propagates from the PAS domain to regulate the C-terminal HAMP and kinase control domains, ultimately modulating a cellular response.IMPORTANCE Pseudomonas aeruginosa is a ubiquitous environmental bacterium and opportunistic pathogen that infects multiple body sites, including the lungs of cystic fibrosis patients. P. aeruginosa senses and responds to its environment via four chemosensory systems. Three of these systems regulate biofilm formation, twitching motility, and chemotaxis. The role of the fourth system, Che2, is unclear but has been implicated in virulence. The Che2 system contains a chemoreceptor called Aer2, which contains a PAS sensing domain that binds heme and senses oxygen. Here, we show that Aer2 uses unprecedented mechanisms to bind O 2 and initiate signaling. These studies provide both the first functional corroboration of the Aer2-PAS signaling mechanism previously proposed from structure as well as a signaling model for Aer2-PAS receptors.KEYWORDS chemoreceptor, PAS domain, signal transduction, Pseudomonas aeruginosa, heme, oxygen P seudomonas aeruginosa is a common environmental bacterium and a significant cause of opportunistic human disease. It survives in complex environments with the aid of 26 chemoreceptors and four chemosensory systems that collectively sense environmental conditions and modify bacterial behavior. The roles of three of these chemosensory systems are known: one modulates type IV pili production and twitching motility (Pil-Chp system), another controls biofilm formation (Wsp system), and a third

Helicobacter pylori infects the stomach of about half of the world's human population, frequently causing chronic inflammation at the origin of several gastric pathologies. One of the most remarkable characteristics of the species is its remarkable genomic plasticity in which homologous recombination (HR) plays a critical role. Here, we analyzed the role of the H. pylori homologue of the AddAB recombination protein. Bioinformatics analysis of the proteins unveils the similarities and differences of the H. pylori AddAB complex with respect to the RecBCD and AddAB complexes from Escherichia coli and Bacillus subtilis, respectively. Helicobacter pylori mutants lacking functional addB or/and addA show the same level of sensitivity to DNA-damaging agents such as UV or irradiation and of deficiency in intrachromosomal RecA-dependent HR. Epistasis analyses of both DNA repair and HR phenotypes, using double and triple recombination mutants, demonstrate that, in H. pylori, AddAB and RecOR complexes define two separate presynaptic pathways with little functional overlap. However, neither of these complexes participates in the RecA-dependent process of transformation of these naturally competent bacteria.