[Molecular bases of virulence in Neisseria gonorrhoeae].
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Pilin
Antigenic variation
Neisseria gonorrhoeae
Phase variation
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The pilus of Neisseria gonorrhoeae (the gonococcus Gc), the causative agent of gonorrhoea, promotes attachment of the gonococcus to the host epithelium and is essential for the establishment of disease. The ability of N. gonorrhoeae to infect previously exposed individuals is partially due to pilus antigenic variation. In addition, variation of the pilus has been proposed to function in the adaptation of the gonococcus to host environments. Previously, we described the development of a competitive reverse transcriptase (RT)-PCR assay that quantifies the frequency of pilin antigenic variation within a gonococcal population. Using this assay, the effect of different biologically relevant environmental conditions on the frequency of pilin antigenic variation was tested. Of the environmental conditions examined in vitro, only limited iron affected a significant change in the frequency of antigenic variation. Further investigation revealed that an observed increase in pilin antigenic variation reflected an increase in other DNA recombination and DNA repair processes within iron-starved cultures. In addition, this low iron-induced increase was determined to be independent of changes in RecA expression and was observed in a Fur mutant strain. As gonococci encounter conditions of low iron during infection, these data suggest that iron-limitation signals for increased recombinational events that are important for gonococcal pathogenesis.
Pilin
Neisseria gonorrhoeae
Antigenic variation
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Summary Pili of Neisseria gonorrhoeae are correlated with Increased bacterial attachment to epithelial cells and undergo both phase and antigenic variation. Phase variation of gonococcal pili can be brought about by recombination events in the pilin structural gene, pilE , or by the on/off switch in expression of PilC, a pilus biogenesis protein for which two loci exist. We have studied the binding to epithelial cell lines and to fixed tissue sections of N. gonorrhoeae MS11 derivatives and mutants carrying structurally defined PilE and PilC proteins, in situ binding studies of N. gonorrhoeae to formalin‐fixed tissue sections resulted in a binding pattern similar to that obtained using viable epithelial cell lines of different origin. Piliated gonococcal clones, containing different pilE sequences, varied dramatically from one another in their efficiencies at binding to corneal and conjunctival tissue, but bound equally well to cervical and endometrial tissues. Further, the binding data suggested that PJIC expression by itself, i.e. without pili, cannot confer bacterial binding and that expression of either PilC1 or PiiC2 does not confer different binding properties to the bacterial cells. Possible receptors for piliated gonococci were expressed in human tissues, such as cervix, endometrium, cornea, intestine, stomach, mid‐brain and meninges, but not in human kidney. Pretreatment of the target tissues with Proteinase K decreased the gonococcal binding dramatically, whereas pretreatment with neuraminidase and meta‐periodate, which cleave carbon‐carbon linkages between vicinal hydroxyl groups in carbohydrates, did not affect attachment of gonococci. These data argue that pilus‐dependent attachment of N. gonorrhoeae to human tissue may be mediated by a eukaryotic receptor having protein characteristics, and that the pilus subunit sequence may play an important role in the interaction with human cornea.
Neisseria gonorrhoeae
Pilin
Phase variation
Fimbriae Proteins
Antigenic variation
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Phase-variable expression of different versions of the same gene, as in the case of opa genes, or of genes that contribute to the structure of the same macromolecule, as occurs with lipooligosaccharide (LOS) biosynthesis genes, results in reversible changes in the antigenic makeup of the bacterial surface. Pilin antigenic variation, the result of new genetic information recombining into the pilin gene, is perhaps the most fascinating example of true antigenic variation in Neisseria gonorrhoeae. Despite the experimental challenges inherent in studying this human-specific pathogen, evidence that variable expression of surface molecules plays a critical role in gonococcal pathogenesis is strong. The depth of variability created by the size of the pilin repertoire and the seemingly random manner by which cassettes are inserted make Neisseria pilus antigenic variation one of the most fascinating stories of genetic diversity in bacterial pathogenesis. The purpose of pilus phase variation in bacterial pathogenesis is less intuitive than that of antigenic variation. Experimental infection of mice may be a useful tool for investigating the kinetics of gonococcal opacity (Opa) expression in vivo. Recovery of Opa-positive variants occurs following vaginal inoculation of mice with a predominantly Opa-negative inoculum. Acquisition of iron for growth and as a cofactor of several key enzymes in the low-iron environment of the host is important for successful colonization by most microbes.
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Neisseria gonorrhoeae
Antigenic variation
Neisseria
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The obligate human pathogen Neisseria gonorrhoeae is the sole aetiologic agent of the sexually transmitted infection, gonorrhea. Required for gonococcal infection, Type IV pili (Tfp) mediate many functions including adherence, twitching motility, defense against neutrophil killing, and natural transformation. Critical for immune escape, the gonococcal Tfp undergoes antigenic variation, a recombination event at the pilE locus that varies the surface exposed residues of the major pilus subunit PilE (pilin) in the pilus fiber. This programmed recombination system has the potential to produce thousands of pilin variants and can produce strains with unproductive pilin molecules that are completely unable to form Tfp. Saturating mutagenesis of the 3' third of the pilE gene identified 68 unique single nucleotide mutations that each resulted in an underpiliated colony morphology. Notably, all isolates, including those with undetectable levels of pilin protein and no observable surface-exposed pili, retained an intermediate level of transformation competence not exhibited in ΔpilE strains. Site-directed, nonsense mutations revealed that only the first 38 amino acids of the mature pilin N-terminus (the N-terminal domain or Ntd) are required for transformation competence, and microscopy, ELISAs and pilus purification demonstrate that extended Tfp are not required for competence. Transformation in strains producing only the pilin Ntd has the same genetic determinants as wild-type transformation. The Ntd corresponds to the alternative product of S-pilin cleavage, a specific proteolysis unique to pathogenic Neisseria. Mutation of the S-pilin cleavage site demonstrated that S-pilin cleavage mediated release of the Ntd is required for competence when a strain produces unproductive pilin molecules that cannot assemble into a Tfp through mutation or antigenic variation. We conclude that S-pilin cleavage evolved as a mechanism to maintain competence in nonpiliated antigenic variants and suggest there are alternate forms of the Tfp assembly apparatus that mediate various functions including transformation.
Pilin
Neisseria gonorrhoeae
Fimbriae Proteins
Phase variation
Antigenic variation
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ABSTRACT Intragenic recombination between the single complete pilin gene (expression locus) and multiple, distinct, partial pilin gene copies (silent, storage loci) is thought to account for the generation of pilus antigenic diversity and piliation phase (on-off) changes exhibited by Neisseria gonorrhoeae. The mechanisms operating in the genomic rearrangements associated with these forms of pilus variation were investigated through the study of isogenic strains of gonococci bearing either wild-type or altered recA alleles. Examination of the rates of pilus phase variation and the genetic basis for changes in piliation status displayed by these strains show that recA mediated homologous recombination is required for these high frequency events and confirm that the nonpiliated state results from mutations in the expressed pilin gene. In a strain that is deficient in recA mediated homologous recombination, pilus phase variation occurs at a 100-1000-fold reduced rate and results predominantly from one class of spontaneous frameshift mutations within the pilin structural gene.
Pilin
Phase variation
Antigenic variation
Neisseria gonorrhoeae
Gene conversion
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Pilin
Neisseria gonorrhoeae
Phase variation
Antigenic variation
Neisseria
Neisseriaceae
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ABSTRACT Variation of the pilus of Neisseria gonorrhoeae occurs by the recombination of silent pilin DNA sequences into the pilin expression locus. We have developed a quantitative, competitive reverse transcription-PCR assay which measures the frequency of pilin antigenic variation independently of changes in gonococcal colony morphology and have determined this frequency within a gonococcal population. We have also studied the frequency of antigenic variation during growth and have concluded that growth does not dramatically influence the frequency of pilin antigenic variation, although a reproducible, twofold increase is observed upon the transition into late log/stationary phase.
Pilin
Neisseria gonorrhoeae
Antigenic variation
Phase variation
Neisseriaceae
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In order to investigate possible functional consequences of phase and antigenic variation of meningococci, the attachment of 15 strains of Neisseria meningitidis to human erythrocytes was studied by a nitrocellulose hemadsorption assay. This assay allows the study of individual meningococcal colonies with respect to erythrocyte attachment. Of the 15 strains studied, 7 demonstrated binding of human erythrocytes (HA+). Among these seven strains, the percentage of colonies that were HA+ ranged from 0.2 to 97%. Meningococcal colonies that did not produce pilin (the major structural subunit of pili) did not demonstrate erythrocyte binding (HA-). The HA+ colony phenotype was correlated with assembly of pilin into pili and expression of pili on the meningococcal surface. However, only some piliated colonies bound human erythrocytes. This could not be explained by differences between piliated HA+ and HA- colonies in the amount of pilin produced or by differences in number of pili expressed per diplococcus. Pili of five of the meningococcal strains with HA+ colonies were antigenically related to gonococcal pili (class I meningococcal pili), but HA+ colonies were also seen in two meningococcal strains expressing class II meningococcal pili. Changes from HA+ to HA- and from HA- to HA+, in the presence of continuing pilin production and pilus assembly, occurred at frequencies of up to 10(-2)/CFU per generation. Such frequencies resemble those of phase and antigenic variation described previously for Neisseria species pilin. These studies indicate that phase variation influences the ability of meningococci to attach to human cells and suggest that meningococci may express functionally different pili.
Pilin
Phase variation
Neisseria gonorrhoeae
Neisseriaceae
Antigenic variation
Fimbriae Proteins
Neisseria
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Pilin
Neisseria gonorrhoeae
Antigenic variation
Phase variation
Neisseria
Fimbriae Proteins
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Citations (348)
Neisseria gonorrhoeae
Antigenic variation
Neisseria
Variation (astronomy)
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