Type I Secretion System

This chapter on mechanisms of genome plasticity in Neisseria meningitidis initially gives a short overview over the genetic variability at the population level and some peculiarities of meningococcal genome organization as revealed by genome sequencing projects. Later, the focus is on genetic mechanisms and genomic features that are paramount for the generation of genomic flexibility, and a brief account of the genetic basis of virulence in Neisseria meningitidis as far as it is known today. Exogenous and endogenous stress induces DNA damage in the meningococcal genome that must be repaired, and DNA repair mechanisms are therefore likely to have a key role in meningococcal genome dynamics. So far, Escherichia coli has served as the prime model organism for DNA repair systems in other microorganisms such as N. meningitidis. The majority of strong mutators found in a number of bacterial species have a defective MMR pathway due to the inactivation of mutS or mutL genes. In addition to global mutation and phase variation, intragenomic as well as intergenomic recombination is of pivotal importance for the generation of genome flexibility in N. meningitidis, and one of the most striking characteristics of the meningococcal genomes is the abundance and diversity of repetitive DNA serving as potential target sites for homologous recombination or replication slippage.

One of the first bacterial species for which N-acylhomoserine lactone (AHL) quorum sensing (QS) was described was Erwinia carotovora. Since then, QS has been well studied in the soft-rot erwinias, where, as described in this chapter, QS plays a key role in the regulation of secreted plant cell wall-degrading enzymes (PCWDEs) production and hence in virulence. In certain strains, a well-defined AHL QS system also controls production of β-lactam antibiotic, carbapenem. In addition, it must be emphasized that QS is only one of many regulatory inputs into virulence factor production in Erwinia. The majority of the key secreted virulence factors of E. carotovora and E. chrysanthemi, including multiple Pels, Peh, Cel, and Svx, are secreted by a type II secretion system known as the Out system. There have been two reports describing the existence of AHL QS in E. amylovora. First, production of a single AHL, most likely 3-oxo-C6-HSL, was described for several Italian strains of E. amylovora; for one strain, production of AHL was observed in planta. Second, AHL activity was detected in the culture supernatant of a Swiss strain of E. amylovora. Both reports describe the detection and partial sequencing of pairs of convergent luxIR homologues, named eamIR.

This chapter provides an overview of how genome sequence data are changing our understanding of the mechanisms and evolution of plant pathogenesis, and discusses the opportunities and challenges genome data present for future research. The majority of plant pathogenic bacteria contain large numbers of genomic islands, and it is clear that in many cases they play an essential role in disease. Genes encoding the P. syringae phytotoxins coronatine and syringomycin reside on PIs, as do their recently discovered counterparts in E. carotovora subsp. atroseptica SCR1043. Pathogenicity and virulence factors have received the greatest focus in the genomic analysis of plant pathogens. Protein secretion systems are essential for pathogenesis in most plant pathogenic bacteria and have been extensively studied, particularly in proteobacterial pathogens. The phytohormones indole-3-acetic acid (IAA or auxin) is produced by many phytopathogenic bacteria. IAA production has been shown to contribute to in planta and epiphytic growth, virulence, and the regulation of syringomycin in P. syringae. Genome analyses of plant pathogens have highlighted three aspects of physiological adaptation to life on plants: specialization, innovation, and flexibility. A largely uninvestigated element in understanding the content and function of plant pathogen genomes, particularly the genomes of facultative plant pathogens, rests on understanding what plant pathogenic bacteria do when not causing disease.

This chapter describes a first putative type I secretion system (Lss) of Legionella pneumophila which is encoded by the lssXYZABD locus. In order to identify substrates of this Lss secretion system, the authors performed comparative two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis of extracellular proteins of L. pneumophila Corby wild type, Corby lssB mutant, and complemented lssB mutant strain. In this study it was shown that the identified VirK protein of L. pneumophila is secreted in an LssB- but not a TatC-dependent manner. Referring to the results, it is implied that the Lss secretion system is also linked to the Lsp type II secretion system in L. pneumophila. It is speculated that VirK and p14 are secreted via a concerted action of the Lss and the Lsp system which includes cleavage of the signal sequences during secretion. In Agrobacterium and Rhizobium VirK is associated with the vir-regulon, and it was postulated that VirK should have a function in the interaction of the bacteria with the host cells. Therefore, the authors started to generate a virK mutant strain of L. pneumophila Corby and an antibody against VirK to further characterize the expression, secretion, and function of VirK of L. pneumophila.

The Escherichia coli hemolysin, earlier referred to as the hemolysin, is the best-characterized repeats in toxin (RTX) secreted by a type I exoprotein secretion system. The E. coli hemolysin is a significant virulence factor in murine models of peritonitis and ascending urinary tract infection, which suggests it is likely to be an important cytotoxin in human, extraintestinal E. coli diseases. Among E. coli or Salmonella strains there are no known examples of strict RTX leukotoxins in which lytic activity is limited to white blood cells. The general gene organization of the Vibrio cholerae RTX locus is similar to that seen with either of the E. coli hly and ehx loci with C, B, and D RTX homologs, clearly indicating it is a member of the RTX family. The hemolysin occurs less frequently in cystitis strains and only rarely among normal fecal strains. Among the extraintestinal E. coli isolates, the hlyCABDgenes were among the first virulence factors localized to unique, tRNA-associated segments of E. coli chromosomes. The hemolysin genes were eventually linked to P-type pilin and cytotoxic necrotizing factor-1 genes. Recent progress with its study has slowed down because of the difficulty in deriving the physical structure of the hemolysin protein or other RTX toxins and establishing its precise cytotoxic mechanism and role in pathogenesis of extraintestinal E. coli disease. Genomic sequencing has revealed that there are additional RTX-like genes found among many different pathogens; perhaps new efforts to discover their functions will aid progress in the RTX toxin field.

Type III secretion system (TTSS) is believed to have originally evolved from the flagellar export system and is now dispersed among a number of both animal- and plant-interacting gram-negative bacteria. The aim of much current research on TTSS is to understand the mechanisms involved in effector secretion/injection and what the effectors are doing inside the host cell. Bacterial pathogens use several different protein secretion pathways to export virulence proteins from the bacterial cytoplasm to their site of action. Chaperones bind to effector proteins in the bacterial cytosol and remain cytosolic following export of their cognate substrate. Secreted proteins exhibiting significant structural similarities to YopB and YopD are present in all TTSSs of animal pathogens but not plant pathogens. Secretion of LcrQ upon cell contact depletes LcrQ from the cytoplasmic compartment and triggers increased transcription of type III genes. Secretion and polymerization of PrgI are required to complete the assembly of the needle complex and type III export apparatus. Two flagellar components, FlhB, an inner membrane protein with a substantial C-terminal cytoplasmic domain, and FliK, the secreted hook-length control protein, are proposed to be involved in switches substrate specificity process. An important question is how far knowledge of flagellar biogenesis can be extrapolated to understand the structure and function of the type III export apparatus. It is obvious that there will be features unique to each system; however, the basic process of transporting substrates across the bacterial membranes appears to be relatively well conserved.