A central feature of Mycobacterium tuberculosis (Mtb) pathogenesis is the ability of Mtb to survive within macrophages (MØ). Despite its critical importance, our appreciation of the interplay between these two cells remains superficial. We employed microarrays to conduct a stepwise dissection of Mtb-MØ interaction during the invasion of resting bone marrow MØ. Contrary to many bacterial pathogens, engagement by MØ receptors without internalization did not alter Mtb gene expression. Subsequently, a high-resolution profile of Mtb invasion-linked gene expression was generated by assaying the Mtb transcriptome at 20 min intervals up to 2 hr postinfection. Transcriptional responses were detected within minutes of phagocytosis, including gene subsets with distinct temporal profiles. Pharmacological manipulation of phagosomal pH and in vitro acid stress studies revealed that vacuole acidification is an important trigger for differential gene expression. Finally, there are marked species-specific differences in the response of Mtb and M. bovis BCG to intraphagosomal cues.

The AvrPtoB type III effector protein is conserved among diverse genera of plant pathogens suggesting it plays an important role in pathogenesis. Here we report that Pseudomonas AvrPtoB acts inside the plant cell to inhibit programmed cell death (PCD) initiated by the Pto and Cf9 disease resistance proteins and, remarkably, the pro-apoptotic mouse protein Bax. AvrPtoB also suppressed PCD in yeast, demonstrating that AvrPtoB functions as a cell death inhibitor across kingdoms. Using truncated AvrPtoB proteins, we identi®ed distinct N-and C-terminal domains of AvrPtoB that are suf®cient for host recognition and PCD inhibition, respectively. We also identi®ed a novel resistance phenotype, Rsb, that is triggered by an AvrPtoB truncation disrupted in the anti-PCD domain. A Pseudomonas syringae pv. tomato DC3000 strain with a chromosomal mutation in the AvrPtoB C-terminus elicited Rsb-mediated immunity in previously susceptible tomato plants and disease was restored when full-length AvrPtoB was expressed in trans. Thus, our results indicate that a type III effector can induce plant susceptibility to bacterial infection by inhibiting host PCD. Keywords: bacterial pathogenesis/effector proteins/plant disease susceptibility/programmed cell death/type III secretion Introduction Pseudomonas syringae pv. tomato DC3000 is a widely studied model plant pathogen that causes disease on tomato and Arabidopsis. DC3000 uses a type III secretion system (TTSS) to directly deliver bacterial effector proteins into the host cell (Galan and Collmer, 1999). Loss-of-function mutations in the TTSS completely abrogate P.syringae disease formation, indicating that effectors are essential agents of P.syringae pathogenesis (Collmer et al., 2000). Genome-wide analyses of P.syringae have de®ned an inventory of type III effectors (Boch et al., 2002;Fouts et al., 2002;Guttman et al., 2002;Petnicki-Ocwieja et al., 2002;Zwiesler-Vollick et al., 2002) and the functions of these effectors are now being examined. Although the role of effector proteins in pathogen virulence is poorly understood, many effectors have been isolated based on their ability to trigger host immunity.In the`gene-for-gene' model of plant immunity, disease resistance is initiated by recognition of a pathogen avirulence (Avr) effector protein by a plant resistance (R) protein. The tomato R protein Pto, a serine/threonine protein kinase, recognizes and directly interacts with DC3000 effector proteins AvrPto and AvrPtoB, and initiates immunity in tomato by characterized and uncharacterized signaling mechanisms Tang et al., 1999;Kim et al., 2002). In this paper, recognition will be de®ned as the physical interaction of Pto with an effector protein.Recognition alone, however, is not suf®cient to elicit immunity. For example, Pto-dependent immunity requires the Prf gene (Salmeron et al., 1996) and presumably other additional factors to signal host immunity (Shirasu and Schulze-Lefert, 2000;Martin et al., 2003).The R gene-mediated plant immune response is characterized by a series of phy...

Summary Following phagocytosis by macrophages, Mycobacterium tuberculosis (Mtb) senses the intracellular environment and remodels its gene expression for growth in the phagosome. We have identified an Acid and Phagosome Regulated (aprABC) locus that is unique to the Mtb complex and whose gene expression is induced during growth in acidic environments in vitro and in macrophages. Using the aprA promoter, we generated a strain that exhibits high levels of inducible fluorescence in response to growth in acidic medium in vitro and in macrophages. aprABC expression is dependent on the two-component regulator phoPR, linking phoPR signaling to pH sensing. Deletion of the aprABC locus causes defects in gene expression that impact aggregation, intracellular growth, and the relative levels of storage and cell wall lipids. We propose a model where phoPR senses the acidic pH of the phagosome and induces aprABC expression to fine-tune processes unique for intracellular adaptation of Mtb complex bacteria.

Many bacterial pathogens of plants and animals use a type III secretion system to deliver diverse virulence-associated 'effector' proteins into the host cell. The mechanisms by which these effectors act are mostly unknown; however, they often promote disease by suppressing host immunity. One type III effector, AvrPtoB, expressed by the plant pathogen Pseudomonas syringae pv. tomato, has a carboxy-terminal domain that is an E3 ubiquitin ligase. Deletion of this domain allows an amino-terminal region of AvrPtoB (AvrPtoB(1-387)) to be detected by certain tomato varieties leading to immunity-associated programmed cell death. Here we show that a host kinase, Fen, physically interacts with AvrPtoB(1-387 )and is responsible for activating the plant immune response. The AvrPtoB E3 ligase specifically ubiquitinates Fen and promotes its degradation in a proteasome-dependent manner. This degradation leads to disease susceptibility in Fen-expressing tomato lines. Various wild species of tomato were found to exhibit immunity in response to AvrPtoB(1-387 )and not to full-length AvrPtoB. Thus, by acquiring an E3 ligase domain, AvrPtoB has thwarted a highly conserved host resistance mechanism.