Summary Variant surface antigens play an important role in the pathogenesis of Plasmodium falciparum malaria. To date, intensive work has mainly focused on the role in parasite virulence of the P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1) encoded by the var multigene family. Two other multigene families coding for STEVOR and RIFIN have recently also been shown to be expressed in the invasive merozoite as well as on the surface of the infected erythrocyte, implicating them as potential parasite virulence factors. Here we report that STEVOR is an erythrocyte-binding protein recognizing Glycophorin C on the red blood cell (RBC) surface. STEVOR expression on the RBC leads to PfEMP1-independent rosette formation, while antibodies targeting STEVOR in the merozoite can effectively inhibit invasion. Our results suggest a novel role of STEVOR in enabling infected erythrocytes at the schizont stage to bind uninfected erythrocytes to form rosettes, thereby protecting released merozoites from immune detection.

Coevolution of intracellular bacterial pathogens and their host cells resulted in the appearance of effector molecules that when translocated into the host cell modulate its function, facilitating bacterial survival within the hostile host environment. Some of these effectors interact with host chromatin and other nuclear components. In this report, we show that the AnkA protein of Anaplasma phagocytophilum, which is translocated into the host cell nucleus, interacts with gene regulatory regions of host chromatin and is involved in downregulating expression of CYBB (gp91 phox ) and other key host defense genes. AnkA effector protein rapidly accumulated in nuclei of infected cells coincident with changes in CYBB transcription. AnkA interacted with transcriptional regulatory regions of the CYBB locus at sites where transcriptional regulators bind. AnkA binding to DNA occurred at regions with high AT contents. Mutation of AT stretches at these sites abrogated AnkA binding. Histone H3 acetylation decreased dramatically at the CYBB locus during A. phagocytophilum infection, particularly around AnkA binding sites. Transcription of CYBB and other defense genes was significantly decreased in AnkA-transfected HL-60 cells. These data suggest a mechanism by which intracellular pathogens directly regulate host cell gene expression mediated by nuclear effectors and changes in host chromatin structure.Intracellular pathogens, through longstanding associations with host cells, evolved mechanisms that promote survival within the often-hostile environment of their hosts (13). Global analysis of mammalian gene expression in response to intracellular bacteria identified major pathways affected during infection (4). However, previous studies largely focused on interactions of bacteria and/or bacterial effectors with the host cell surface and cytoplasmic signaling pathways (19). Transfer of bacterial effector proteins into the eukaryotic host cell cytoplasm is also a recognized mechanism of bacterial control over host cells for organisms such as Yersinia (29), Shigella (1), and Listeria (27). In contrast, mechanisms by which bacterial proteins enter the host cell and directly alter gene transcription are underinvestigated, and only a few bacterial molecules that enter the eukaryotic host nucleus are reported. Cytolethal distending toxin, a tripartite toxin produced by a number of bacteria, is translocated into the host cell nucleus and by exerting DNA damage leads to G 2 /M cell cycle arrest, cellular distention, and nuclear enlargement in intoxicated cells (17). Similarly, some proteins of the Amoeba proteus X-bacterium symbiont translocate into the nucleus and induce S-adenosylmethionine synthetase expression (22), whereas T-DNA from Agrobacterium tumefaciens is transported into the nucleus and induces plant cell transformation (20). Additionally, Shigella flexneri OspF is injected into host cells and alters host gene transcription by targeting chromatin access for the transcription factor NF-B (2). Using Anaplasma phagocytophilum...

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Most β-lactam antibiotics are ineffective against Mycobacterium tuberculosis due to the microbe’s innate resistance. The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains has prompted interest to repurpose this class of drugs. To identify the genetic determinants of innate β-lactam resistance, we carried out a synthetic lethality screen on a transposon mutant library for susceptibility to imipenem, a carbapenem β-lactam antibiotic. Mutations in 74 unique genes demonstrated synthetic lethality. The majority of mutations were in genes associated with cell wall biosynthesis. A second quantitative real-time PCR (qPCR)-based synthetic lethality screen of randomly selected mutants confirmed the role of cell wall biosynthesis in β-lactam resistance. The global transcriptional response of the bacterium to β-lactams was investigated, and changes in levels of expression of cell wall biosynthetic genes were identified. Finally, we validated these screens in vivo using the MT1616 transposon mutant, which lacks a functional acyl-transferase gene. Mice infected with the mutant responded to β-lactam treatment with a 100-fold decrease in bacillary lung burden over 4 weeks, while the numbers of organisms in the lungs of mice infected with wild-type bacilli proliferated. These findings reveal a road map of genes required for β-lactam resistance and validate synthetic lethality screening as a promising tool for repurposing existing classes of licensed, safe, well-characterized antimicrobials against tuberculosis.