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Commensal bacteria comprise a large part of the microbial world, playing important roles in human development, health and disease. However, little is known about the genomic content of commensals or how related they are to their pathogenic counterparts. The genus Neisseria, containing both commensal and pathogenic species, provides an excellent opportunity to study these issues. We undertook a comprehensive sequencing and analysis of human commensal and pathogenic Neisseria genomes. Commensals have an extensive repertoire of virulence alleles, a large fraction of which has been exchanged among Neisseria species. Commensals also have the genetic capacity to donate DNA to, and take up DNA from, other Neisseria. Our findings strongly suggest that commensal Neisseria serve as reservoirs of virulence alleles, and that they engage extensively in genetic exchange.

Neisseria gonorrhoeae is the bacterium that causes gonorrhea, a major sexually transmitted disease and a significant cofactor for human immunodeficiency virus transmission. The retactile N. gonorrhoeae type IV pilus (Tfp) mediates twitching motility and attachment. Using live-cell microscopy, we reveal for the first time the dynamics of twitching motility by N. gonorrhoeae in its natural environment, human epithelial cells. Bacteria aggregate into microcolonies on the cell surface and induce a massive remodeling of the microvillus architecture. Surprisingly, the microcolonies are motile, and they fuse to form progressively larger structures that undergo rapid reorganization, suggesting that bacteria communicate with each other during infection. As reported, actin plaques form beneath microcolonies. Here, we show that cortical plaques comigrate with motile microcolonies. These activities are dependent on pilT, the Tfp retraction locus. Cultures infected with a pilT mutant have significantly higher numbers of apoptotic cells than cultures infected with the wild-type strain. Inducing pilT expression with isopropyl-␤-D-thiogalactopyranoside partially rescues cells from infection-induced apoptosis, demonstrating that Tfp retraction is intrinsically cytoprotective for the host. Tfp-mediated attachment is therefore a continuum of microcolony motility and force stimulation of host cell signaling, leading to a cytoprotective effect.Type IV pili (Tfp) are filamentous appendages expressed by a wide range of bacteria, including Synechocystis spp., Pseudomonas aeruginosa, Myxococcus xanthus, Xylella fastidiosa, Clostridium perfringens, enterohaemorrhagic and enteropathogenic Escherichia coli, Neisseria meningitidis, and Neisseria gonorrhoeae (29,32,37,50,54,56,61). The Tfp of several of these bacteria are known to retract, and retraction underlies twitching motility, DNA uptake, phage sensitivity, and social behavior, such as fruiting-body and biofilm formation.N. gonorrhoeae, the bacterium that causes gonorrhea, expresses multiple nonpolar retractile Tfp that promote attachment to epithelial cells (38). Retraction requires the ATPase PilT, which is proposed to disassemble the Tfp fiber (2,13,15). N. gonorrhoeae pilT null mutants express nonretractile Tfp (38,60). Cycles of Tfp extension, substrate tethering, and retraction enable N. gonorrhoeae to crawl on a glass coverslip or agar surface (twitching motility) (34). Twitching motility on epithelial cells has not been studied. Understanding N. gonorrhoeae motility behavior in this environment is important, as the bacterium infects only humans and cannot survive outside the human body.Mechanical forces of 50 to 100 pN are generated by the retraction of a Tfp fiber (24, 38). Thus, Tfp retraction allows N. gonorrhoeae to pull and exert physical stress on its substrate. Tfp retraction induces a number of responses in the infected epithelial cell. It triggers Ca 2ϩ and stress kinase signaling, cytoskeletal rearrangements, and cortical plaque formation and regulates epithelial gen...

BackgroundAdequate termination of an immune response is as important as the induction of an appropriate response. CD46, a regulator of complement activity, promotes T cell activation and differentiation towards a regulatory Tr1 phenotype. This Tr1 differentiation pathway is defective in patients with MS, asthma and rheumatoid arthritis, underlying its importance in controlling T cell function and the need to understand its regulatory mechanisms. CD46 has two cytoplasmic tails, Cyt1 and Cyt2, derived from alternative splicing, which are co-expressed in all nucleated human cells. The regulation of their expression and precise functions in regulating human T cell activation has not been fully elucidated.Methodology/Principal FindingsHere, we first report the novel role of CD46 in terminating T cell activation. Second, we demonstrate that its functions as an activator and inhibitor of T cell responses are mediated through the temporal processing of its cytoplasmic tails. Cyt1 processing is required to turn T cell activation on, while processing of Cyt2 switches T cell activation off, as demonstrated by proliferation, CD25 expression and cytokine secretion. Both tails require processing by Presenilin/γSecretase (P/γS) to exert these functions. This was confirmed by expressing wild-type Cyt1 and Cyt2 tails and uncleavable mutant tails in primary T cells. The role of CD46 tails was also demonstrated with T cells expressing CD19 ectodomain-CD46 C-Terminal Fragment (CTF) fusions, which allowed specific triggering of each tail individually.Conclusions/SignificanceWe conclude that CD46 acts as a molecular rheostat to control human T cell activation through the regulation of processing of its cytoplasmic tails.

Salmonella typhimurium makes cobalamin (vitamin B 12 ) during anaerobic growth (17). Most of the cobalamin synthetic genes are organized into a single operon (cob or cbi) mapping at min 41 (3, 18). The genetic map of the cob operon can be divided into three distinct regions, each of which includes mutations with characteristic phenotypes. Part I has 17 genes (cbi) needed for cobinamide (Cbi) synthesis; strains with mutations in this region (CobI) can synthesize vitamin B 12 when Cbi is provided. Part II of the operon includes mutations that prevent vitamin B 12 synthesis unless 5,6-dimethylbenzimidazole (DMB) is provided (CobII), suggesting that this region contains the genes for DMB synthesis. Part III includes genes for joining Cbi and DMB to produce functional adenosyl-cobalamin (Ado-B 12 ); these mutants (CobIII) cannot make vitamin B 12 even when both DMB and Cbi are provided. The standard view of the pathway is depicted in Fig. 1.A fine-structure genetic map of the cob or cbi operon shows that the three types of mutations are ordered as parts I, III, and II in a single operon (3,18). The nucleotide sequences of parts I and III have been determined (27). On the basis of sequence homology to genes of known function in Pseudomonas denitrificans (for a recent review, see reference 2), it has been possible to assign functions to many of the genes in parts I and III of the cob operon. Mutations in the CobIII region have been analyzed genetically and biochemically, demonstrating that the cobU and cobS open reading frames correspond to part III of the genetic map (11,24,35). These two genes have been assigned the functions diagrammed in Fig.

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The genus Neisseria contains at least eight commensal and two pathogenic species. According to the Neisseria phylogenetic tree, commensals are basal to the pathogens. N. elongata, which is at the opposite end of the tree from N. gonorrhoeae, has been observed to be fimbriated, and these fimbriae are correlated with genetic competence in this organism. We tested the hypothesis that the fimbriae of N. elongata are Type IV pili (Tfp), and that Tfp functions in genetic competence. We provide evidence that the N. elongata fimbriae are indeed Tfp. Tfp, as well as the DNA Uptake Sequence (DUS), greatly enhance N. elongata DNA transformation. Tfp allows N. elongata to make intimate contact with N. gonorrhoeae and to mediate the transfer of antibiotic resistance markers between these two species. We conclude that Tfp functional for genetic competence is a trait of a commensal member of the Neisseria genus. Our findings provide a mechanism for the horizontal gene transfer that has been observed among Neisseria species.

SummaryCatabolite gene activator protein (CAP) is essential for the expression of Pap pili by uropathogenic Escherichia coli. Both in vitro and in vivo analyses indicate that binding of cAMP±CAP centred at 215.5 bp upstream of the papBA promoter is essential for activation of transcription. CAP-dependent activation of papBA requires binding of the leucineresponsive regulatory protein (Lrp) at binding sites that extend from 2180 to 2149 relative to the start site of papBA. Our data indicate that CAP and Lrp bind independently to their respective pap DNA sites. Activation of papBA transcription was eliminated by mutations in the activating region 1 (AR1) of CAP, but not in the AR2 region, similar to class I CAPdependent promoters. Also, like class I promoters, the C-terminal domain of the a-subunit of RNA polymerase appears to play a role in transcription activation. Moreover, phase variation is strictly dependent upon the helical phase of the CAP DNA binding site with respect to the papBA transcription start site. Using an`oriented heterodimer' approach with wild-type and AR1 mutant CAPs, it was shown that the AR1 region of the CAP subunit proximal to papBA is required for stimulation of papBA transcription, whereas AR1 of the promoter-distal subunit is not. Previously, CAP was hypothesized to activate pap transcription indirectly by disrupting repression mediated by H-NS. The results presented here show that AR1 of the promoter-proximal CAP subunit was required for papBA transcription even in the absence of the histone-like protein H-NS. These results show that the promoter-proximal subunit of CAP, bound 215.5 bp upstream of the papBA transcription start site, plays an active role in stimulating papBA transcription, possibly by interacting with the Cterminal domain of the a-subunit of RNA polymerase.