Type IV pilus biogenesis, twitching motility, and DNA uptake in Thermus thermophilus: discrete roles of antagonistic ATPases PilF, PilT1, and PilT2

Identification of subcomplexes and protein-protein interactions in the DNA transporter of Thermus thermophilus HB27

The natural transformation system of the thermophilic bacterium Thermus thermophilus comprises at least 16 competence proteins. Recently we found that the outer membrane (OM) competence protein PilW interacts with the secretin channel, which guides type IV pili (T4P) and potential DNA transporter pseudopili through the OM. Here we have used biochemical techniques to study the interactions of cytoplasmic, inner membrane (IM) and OM components of the DNA transporter in T. thermophilus. We report that PilW is part of a heteropolymeric complex comprising of the cytoplasmic PilM protein, IM proteins PilN, PilO, PilC and the secretin PilQ. Co-purification studies revealed that PilO directly interacts with PilW. In vitro affinity co-purification studies using His-tagged PilC led to the detection of PilC-, PilW-, PilN- and PilO-containing complexes. PilO was identified as direct interaction partner of the polytopic IM protein PilC. PilC was also found to directly interact with the cytoplasmic T4P disassembly ATPase PilT1 thereby triggering PilT1 ATPase activity. This, together with the detection of heteropolymeric PilC complexes which contain PilT1 and the pilins PilA2, PilA4 and PilA5 is in line with the hypothesis that PilC connects the depolymerization ATPase to the base of the pili possibly allowing energy transduction for disassembly of the pilins.

DNA binding by pilins and their interaction with the inner membrane platform of the DNA transporter in Thermus thermophilus

The natural transformation system of Thermus thermophilus has become a model system for studies of the structure and function of DNA transporter in thermophilic bacteria. The DNA transporter in T. thermophilus is functionally linked to type IV pili (T4P) and the major pilin PilA4 plays an essential role in both systems. However, T4P are dispensable for natural transformation. In addition to pilA4, T. thermophilus has a gene cluster encoding the three additional pilins PilA1-PilA3; deletion of the cluster abolished natural transformation but retained T4P biogenesis. In this study, we investigated the roles of single pilins PilA1, PilA2 and PilA3 in natural transformation by mutant studies. These studies revealed that each of these pilins is essential for natural transformation. Two of the pilins, PilA1 and PilA2, were found to bind dsDNA. PilA1 and PilA3 were detected in the inner membrane (IM) but not in the outer membrane (OM) whereas PilA2 was present in both membranes. All three pilins where absent in pilus fractions. This suggests that the pilins form a short DNA binding pseudopilus anchored in the IM. PilA1 was found to bind to the IM assembly platform of the DNA transporter via PilM and PilO. These data are in line with the hypothesis that a DNA binding pseudopilus is connected via an IM platform to the cytosolic motor ATPase PilF.

Natural transformation in Gram-negative bacteria thriving in extreme environments: from genes and genomes to proteins, structures and regulation

Extremophilic prokaryotes live under harsh environmental conditions which require far-reaching cellular adaptations. The acquisition of novel genetic information via natural transformation plays an important role in bacterial adaptation. This mode of DNA transfer permits the transfer of genetic information between microorganisms of distant evolutionary lineages and even between members of different domains. This phenomenon, known as horizontal gene transfer (HGT), significantly contributes to genome plasticity over evolutionary history and is a driving force for the spread of fitness-enhancing functions including virulence genes and antibiotic resistances. In particular, HGT has played an important role for adaptation of bacteria to extreme environments. Here, we present a survey of the natural transformation systems in bacteria that live under extreme conditions: the thermophile Thermus thermophilus and two desiccation-resistant members of the genus Acinetobacter such as Acinetobacter baylyi and Acinetobacter baumannii. The latter is an opportunistic pathogen and has become a world-wide threat in health-care institutions. We highlight conserved and unique features of the DNA transporter in Thermus and Acinetobacter and present tentative models of both systems. The structure and function of both DNA transporter are described and the mechanism of DNA uptake is discussed.

Functional dissection of structural regions of the Thermus thermophilus competence protein PilW: Implication in secretin complex stability, natural transformation and pilus functions

Uptake of DNA from the environment into the bacterial cytoplasm is mediated by a macromolecular transport machinery that is similar in structure and function to type IV pili (T4P) and, indeed, DNA translocator and T4P share common components. One is the secretin PilQ which is assembled into homopolymeric complexes forming highly dynamic outer membrane (OM) channels mediating pilus extrusion and DNA uptake. How PilQ interacts with the motor is still enigmatic. Here, we have used biochemical and genetic techniques to study the interaction of PilQ with PilW, a unique protein which is essential for natural transformation and T4P extrusion of T. thermophilus. PilQ and PilW form high molecular mass complexes in the OM of T. thermophilus. When pilW was deleted, PilQ complexes were no longer observed but only PilQ monomers, accompanied by a loss of DNA uptake as well as a loss of T4P and twitching motility. Piliation of a ΔpilT2/ΔpilW double mutant suggests that PilW is important for stable assembly of PilQ complexes. To analyze the role of different regions of PilW, partial deletions (pilW_∆2-40, pilW_∆50-150, pilW_∆163-216 and pilW_∆216-292) were generated and the effect on DNA uptake, PilQ complex formation and T4P functions such as twitching motility, biofilm formation and cell-cell interaction was studied. These studies revealed that a central disordered region in PilW is required for pilus dynamics. We propose that PilW is part of a protein network that connects the transport ATPase to drive different functions of the DNA translocator and T4P.

Cyclic-di-GMP and ADP bind to separate domains of PilB as mutual allosteric effectors

PilB is the assembly ATPase for the bacterial type IV pilus (T4P), and as a consequence, it is essential for T4P-mediated bacterial motility. In some cases, PilB has been demonstrated to regulate the production of exopolysaccharide (EPS) during bacterial biofilm development independently of or in addition to its function in pilus assembly. While the ATPase activity of PilB resides at its C-terminal region, the N terminus of a subset of PilBs forms a novel cyclic-di-GMP (cdG)-binding domain. This multi-domain structure suggests that PilB binds cdG and adenine nucleotides through separate domains which may influence the functionality of PilB in both motility and biofilm development. Here, Chloracidobacterium thermophilum PilB is used to investigate ligand binding by its separate domains and by the full-length protein. Our results confirm the specificity of these individual domains for their respective ligands and demonstrate communications between these domains in the full-length protein. It is clear that when the N- and the C-terminal domains of PilB bind to cdG and ADP, respectively, they mutually influence each other in conformation and in their binding to ligands. We propose that the interactions between these domains in response to their ligands play critical roles in modulating or controlling the functions of PilB as a regulator of EPS production and as the T4P assembly ATPase.

Cryo-electron microscopy reveals two distinct type IV pili assembled by the same bacterium

Type IV pili are flexible filaments on the surface of bacteria, consisting of a helical assembly of pilin proteins. They are involved in bacterial motility (twitching), surface adhesion, biofilm formation and DNA uptake (natural transformation). Here, we use cryo-electron microscopy and mass spectrometry to show that the bacterium Thermus thermophilus produces two forms of type IV pilus ('wide' and 'narrow'), differing in structure and protein composition. Wide pili are composed of the major pilin PilA4, while narrow pili are composed of a so-far uncharacterized pilin which we name PilA5. Functional experiments indicate that PilA4 is required for natural transformation, while PilA5 is important for twitching motility.

NMR resonance assignments for the GSPII-C domain of the PilF ATPase from Thermus thermophilus in complex with c-di-GMP

The natural transformation system of the thermophilic bacterium Thermus thermophilus is one of the most efficient DNA transport systems in terms of DNA uptake rate and promiscuity. The DNA transporter of T. thermophilus plays an important role in interdomain DNA transfer in hot environments. PilF is the traffic ATPase that provides the energy for the assembly of the DNA translocation machinery and the functionally linked type IV pilus system in T. thermophilus. In contrast to other known traffic ATPases, the N-terminal region of PilF harbors three consecutive domains with homology to general secretory pathway II (GSPII) domains. These GSPII-like domains influence pilus assembly, twitching motility and transformation efficiency. A structural homolog of the PilF GSPII-like domains, the N-terminal domain of the traffic ATPase MshE from Vibrio cholerae, was recently crystallized in complex with the bacterial second messenger c-di-GMP. In order to study the consequences of c-di-GMP binding on the three-dimensional architecture of PilF, we initiated structural studies on the PilF GSPII-like domains. Here, we present the ¹H, ¹³C and ¹⁵N chemical shift assignments for the isolated PilF GSPII-C domain from T. thermophilus in complex with c-di-GMP. In addition, the structural dynamics of the complex was investigated in an {¹H},¹⁵N-hetNOE experiment.

NMR resonance assignments for the GSPII-B domain of the traffic ATPase PilF from Thermus thermophilus in the apo and the c-di-GMP-bound state

The PilF protein from the thermophilic bacterium Thermus thermophilus is a traffic ATPase powering the assembly of the DNA translocation machinery as well as of type 4 pili. Thereby PilF mediates the natural transformability of T. thermophilus. PilF contains a C-terminal ATPase domain and three N-terminal domains with partial homology to so-called general secretory pathway II (GSPII) domains. These three GSPII domains (GSPII-A, GSPII-B and GSPII-C) are essential for pilus assembly and twitching motility. They show varying degrees of sequence homology to the N-terminal domain of the ATPase MshE from Vibrio cholerae which binds the bacterial second messenger molecule c-di-GMP. NMR experiments demonstrate that the GSPII-B domain of PilF also binds c-di-GMP with high affinity and forms a 1:1 complex in slow exchange on the NMR time scale. As a prerequisite for structural studies of c-di-GMP binding to the GSPII-B domain of T. thermophilus PilF we present here the NMR resonance assignments for the apo and the c-di-GMP bound state of GSPII-B. In addition, we map the binding site for c-di-GMP on the GSPII-B domain using chemical shift perturbation data and compare the dynamics of the apo and the c-di-GMP-bound state of the GSPII-B domain based on {¹H},¹⁵N-hetNOE data.

Structure and Properties of a Natural Competence-Associated Pilin Suggest a Unique Pilus Tip-Associated DNA Receptor

Natural competence is the term used to describe the uptake of "naked" extracellular DNA by bacteria; it plays a significant role in horizontal genetic exchange. It is associated with type IV pili, and specialized competence pili mediate DNA uptake. Here, we show that the crystal structure of a competence-associated protein from Thermus thermophilus, ComZ, consists of a type II secretion pseudopilin-like domain, with a large β-solenoid domain inserted into the β-sheet of the pilin-like fold. ComZ binds with high affinity to another competence-associated pilin, PilA2, which lies adjacent to the comZ gene in the genome. The crystal structure of PilA2 revealed a similar type II secretion pseudopilin-like fold, with a small subdomain; docking simulations predicted that PilA2 binds between the pseudopilin-like and β-solenoid domains of ComZ. Electrophoretic shift analysis and DNase protection studies were used to show that ComZ alone and the ComZ/PilA2 complex are able to bind DNA. Protection against reductive dimethylation was used in combination with mass spectrometry and site-directed mutagenesis to identify two lysine residues in ComZ which are involved in DNA binding. They are located between the two domains in ComZ, on the opposite side from the predicted PilA2 binding site. These results suggest a model in which PilA2 assists ComZ in forming the competence pilus tip and DNA binds to the side of the fiber. The results demonstrate how a type IV pilin can be adapted to a specific function by domain insertion and provide the first structural insights into a tip-located competence pilin.IMPORTANCE Thermus thermophilus is a thermophilic bacterium which is capable of natural transformation, the uptake of external DNA with high efficiency. DNA uptake is thought to be mediated by a competence-associated pilus, which binds the DNA substrate and mediates its transfer across the outer membrane and periplasm. Here, we describe the structural and functional analysis of two pilins which are known to be essential for DNA uptake, ComZ and PilA2. ComZ adopts an unusual structure, incorporating a large β-solenoid domain into the pilin structural framework. We argue on structural grounds that this structure cannot readily be accommodated into the competence pilus fiber unless it is at the tip. We also show that ComZ binds DNA and identify two lysine residues which appear to be important for DNA binding. These results suggest a model in which ComZ and PilA2 form a tip-associated DNA receptor which mediates DNA uptake.

Selection-free markerless genome manipulations in the polyploid bacterium Thermus thermophilus

A genome manipulation approach based on double-crossover homologous recombination was developed in the polyploid model organism Thermus thermophilus HB27 without the use of any selectable marker. The method was established and optimized by targeting the megaplasmid-encoded β-glucosidase gene bgl. When linear and supercoiled forms of marker-free suicide vector were used for transformations, the frequencies of obtaining apparent Bgl^- mutant were 10^- 5 and 10^- 3, respectively; while the frequency could reach 10^- 2 when transformation with concatemer form of the same vector. All randomly selected Bgl^- colonies from the transformations were found to be true bgl knockout mutants. Thus, markerless gene deletion mutants could be constructed in T. thermophilus by the direct selection-free method. The functionality of this approach was further demonstrated by deletion of one chromosomal locus (TTC_0340-0341) as well as by generation of a reporter strain for the phytoene synthase promoter (PcrtB), homozygous mutants of the both targets could also be detected with a frequency of approximately 10^- 2. During the genome modification process, heterozygous cells carrying two different alleles at a same locus (e.g., bgl and pyrE) could also be generated. However, in the absence of selection pressure, these strains could rapidly convert to homozygous strains containing only one of the two alleles. This indicated that allele segregation could occur in the heterozygous T. thermophilus cells, which probably explained the ease of obtaining homozygous gene deletion mutants with high frequency (10^- 2) in the polyploid genomic background, as after the mutant allele had been introduced to the target region, allele segregation would lead to homozygous mutant cells. This marker-free genome manipulation approach does not require phenotype-based screens, and is applicable in gene deletion and tagging applications.

Twitch or swim: towards the understanding of prokaryotic motion based on the type IV pilus blueprint

Bacteria and archaea are evolutionarily distinct prokaryotes that diverged from a common ancestor billions of years ago. However, both bacteria and archaea assemble long, helical protein filaments on their surface through a machinery that is conserved at its core. In both domains of life, the filaments are required for a diverse array of important cellular processes including cell motility, adhesion, communication and biofilm formation. In this review, we highlight the recent structures of both the type IV pilus machinery and the archaellum determined in situ. We describe the current level of functional understanding and discuss how this relates to the pressures facing bacteria and archaea throughout evolution.

The traffic ATPase PilF interacts with the inner membrane platform of the DNA translocator and type IV pili from Thermus thermophilus

A major driving force for the adaptation of bacteria to changing environments is the uptake of naked DNA from the environment by natural transformation, which allows the acquisition of new capabilities. Uptake of the high molecular weight DNA is mediated by a complex transport machinery that spans the entire cell periphery. This DNA translocator catalyzes the binding and splitting of double‐stranded DNA and translocation of single‐stranded DNA into the cytoplasm, where it is recombined with the chromosome. The thermophilic bacterium Thermus thermophilus exhibits the highest transformation frequencies reported and is a model system to analyze the structure and function of this macromolecular transport machinery. Transport activity is powered by the traffic ATPase PilF, a soluble protein that forms hexameric complexes. Here, we demonstrate that PilF physically binds to an inner membrane assembly platform of the DNA translocator, comprising PilMNO, via the ATP‐binding protein PilM. Binding to PilMNO or PilMN stimulates the ATPase activity of PilF ~ 2‐fold, whereas there is no stimulation when binding to PilM or PilN alone. A PilM_K26A variant defective in ATP binding still binds PilF and, together with PilN, stimulates PilF‐mediated ATPase activity. PilF is unique in having three conserved GSPII (general secretory pathway II) domains (A–C) at its N terminus. Deletion analyses revealed that none of the GSPII domains is essential for binding PilMN, but GSPIIC is essential for PilMN‐mediated stimulation of ATP hydrolysis by PilF. Our data suggest that PilM is a coupling protein that physically and functionally connects the soluble motor ATPase PilF to the DNA translocator via the PilMNO assembly platform.

Pull in and Push Out: Mechanisms of Horizontal Gene Transfer in Bacteria

Horizontal gene transfer (HGT) plays an important role in bacterial evolution. It is well accepted that DNA is pulled/pushed into recipient cells by conserved membrane-associated DNA transport systems, which allow the entry of only single-stranded DNA (ssDNA). However, recent studies have uncovered a new type of natural bacterial transformation in which double-stranded DNA (dsDNA) is taken up into the cytoplasm, thus complementing the existing methods of DNA transfer among bacteria. Regulated by the stationary-phase regulators RpoS and cAMP receptor protein (CRP), Escherichia coli establishes competence for natural transformation with dsDNA, which occurs in agar plates. To pass across the outer membrane, a putative channel, which may compete for the substrate with the porin OmpA, may mediate the transfer of exogenous dsDNA into the cell. To pass across the inner membrane, dsDNA may be bound to the periplasmic protein YdcS, which delivers it into the inner membrane channel formed by YdcV. The discovery of cell-to-cell contact-dependent plasmid transformation implies the presence of additional mechanism(s) of transformation. This review will summarize the current knowledge about mechanisms of HGT with an emphasis on recent progresses regarding non-canonical mechanisms of natural transformation. Fully understanding the mechanisms of HGT will provide a foundation for monitoring and controlling multidrug resistance.

Functional dissection of the three N-terminal general secretory pathway domains and the Walker motifs of the traffic ATPase PilF from Thermus thermophilus

The traffic ATPase PilF of Thermus thermophilus powers pilus assembly as well as uptake of DNA. PilF differs from other traffic ATPases by a triplicated general secretory pathway II, protein E, N-terminal domain (GSPIIABC). We investigated the in vivo and in vitro roles of the GSPII domains, the Walker A motif and a catalytic glutamate by analyzing a set of PilF deletion derivatives and pilF mutants. Here, we report that PilF variants devoid of the first two or all three GSPII domains do not form stable hexamers indicating a role of the triplicated GSPII domain in complex formation and/or stability. A pilFΔGSPIIC mutant was significantly impaired in piliation which leads to the conclusion that the GSPIIC domain plays a vital role in pilus assembly. Interestingly, the pilFΔGSPIIC mutant was hypertransformable. This suggests that GSPIIC strongly affects transformation efficiency. A pilF∆GSPIIA mutant exhibited wild-type piliation but reduced pilus-mediated twitching motility, suggesting that GSPIIA plays a role in pilus dynamics. Furthermore, we report that pilF mutants with a defect in the ATP binding Walker A motif or in the catalytic glutamate residue are defective in piliation and natural transformation. These findings show that both, ATP binding and hydrolysis, are essential for the dual function of PilF in natural transformation and pilus assembly.

Topology and Structure/Function Correlation of Ring- and Gate-forming Domains in the Dynamic Secretin Complex of Thermus thermophilus

Secretins are versatile outer membrane pores used by many bacteria to secrete proteins, toxins, or filamentous phages; extrude type IV pili (T4P); or take up DNA. Extrusion of T4P and natural transformation of DNA in the thermophilic bacterium Thermus thermophilus requires a unique secretin complex comprising six stacked rings, a membrane-embedded cone structure, and two gates that open and close a central channel. To investigate the role of distinct domains in ring and gate formation, we examined a set of deletion derivatives by cryomicroscopy techniques. Here we report that maintaining the N0 ring in the deletion derivatives led to stable PilQ complexes. Analyses of the variants unraveled that an N-terminal domain comprising a unique βββαβ fold is essential for the formation of gate 2. Furthermore, we identified four βαββα domains essential for the formation of the N2 to N5 rings. Mutant studies revealed that deletion of individual ring domains significantly reduces piliation. The N1, N2, N4, and N5 deletion mutants were significantly impaired in T4P-mediated twitching motility, whereas the motility of the N3 mutant was comparable with that of wild-type cells. This indicates that the deletion of the N3 ring leads to increased pilus dynamics, thereby compensating for the reduced number of pili of the N3 mutant. All mutants exhibit a wild-type natural transformation phenotype, leading to the conclusion that DNA uptake is independent of functional T4P.

Strategies to Block Bacterial Pathogenesis by Interference with Motility and Chemotaxis

Infections by motile, pathogenic bacteria, such as Campylobacter species, Clostridium species, Escherichia coli, Helicobacter pylori, Listeria monocytogenes, Neisseria gonorrhoeae, Pseudomonas aeruginosa, Salmonella species, Vibrio cholerae, and Yersinia species, represent a severe economic and health problem worldwide. Of special importance in this context is the increasing emergence and spread of multidrug-resistant bacteria. Due to the shortage of effective antibiotics for the treatment of infections caused by multidrug-resistant, pathogenic bacteria, the targeting of novel, virulence-relevant factors constitutes a promising, alternative approach. Bacteria have evolved distinct motility structures for movement across surfaces and in aqueous environments. In this review, I will focus on the bacterial flagellum, the associated chemosensory system, and the type-IV pilus as motility devices, which are crucial for bacterial pathogens to reach a preferred site of infection, facilitate biofilm formation, and adhere to surfaces or host cells. Thus, those nanomachines constitute potential targets for the development of novel anti-infectives that are urgently needed at a time of spreading antibiotic resistance. Both bacterial flagella and type-IV pili (T4P) are intricate macromolecular complexes made of dozens of different proteins and their motility function relies on the correct spatial and temporal assembly of various substructures. Specific type-III and type-IV secretion systems power the export of substrate proteins of the bacterial flagellum and type-IV pilus, respectively, and are homologous to virulence-associated type-III and type-II secretion systems. Accordingly, bacterial flagella and T4P represent attractive targets for novel antivirulence drugs interfering with synthesis, assembly, and function of these motility structures.

Structure of a type IV pilus machinery in the open and closed state

Proteins of the secretin family form large macromolecular complexes, which assemble in the outer membrane of Gram-negative bacteria. Secretins are major components of type II and III secretion systems and are linked to extrusion of type IV pili (T4P) and to DNA uptake. By electron cryo-tomography of whole Thermus thermophilus cells, we determined the in situ structure of a T4P molecular machine in the open and the closed state. Comparison reveals a major conformational change whereby the N-terminal domains of the central secretin PilQ shift by ∼30 Å, and two periplasmic gates open to make way for pilus extrusion. Furthermore, we determine the structure of the assembled pilus.

DOI:http://dx.doi.org/10.7554/eLife.07380.001

Gram-negative bacteria can cause serious diseases in humans, such as cholera and bacterial meningitis. These bacteria are surrounded by two membranes: an inner membrane and an outer membrane. Proteins called secretins are components of several large molecular complexes that are embedded within the outer membrane. Some secretin-containing complexes form pores in the bacterial membranes and allow molecules to pass in or out of the cell.

Some secretins also form part of the machinery that allow Gram-negative bacteria to grow fibre-like structures called type IV pili. These pili help bacteria that cause infections to move and stick to host cells, where they can also trigger massive changes in the host cells' architecture. Multiple copies of a secretin protein called PilQ form a channel in the outer membrane of the bacteria that allows a type IV pilus to grow out of the surface of the cell. The pilus can then hook the bacteria onto surfaces and other cells. There is evidence to suggest the type IV pilus machinery is involved in the uptake of DNA from other bacteria, an important but poorly understood process that has contributed to the spread of multi-drug resistance.

Now, Gold et al. have used a cutting-edge technique called ‘electron cryo-tomography’ to analyse the three-dimensional structure of the machinery that builds the type IV pili in the membranes of a bacterium called Thermus thermophilus. This analysis revealed that, similar to many other channel complexes, the PilQ channel can be ‘open’ or ‘closed’. When pili are absent, the channel is closed, but the channel opens when pili are present. Further analysis also revealed the structure of an assembled pilus.

Next, Gold et al. studied the open state of the type IV pilus in more detail and observed that a region of each of the PilQ proteins moves a considerable distance to make way for the pilus to enter the central pore. These results will pave the way for future studies of type IV pili and other secretin-containing complexes and underpin efforts to investigate new drug targets to combat bacterial infections.

DOI:http://dx.doi.org/10.7554/eLife.07380.002

The Thermus thermophilus comEA/comEC operon is associated with DNA binding and regulation of the DNA translocator and type IV pili

Natural transformation systems and type IV pili are linked in many naturally competent bacteria. In the Gram-negative bacterium Thermus thermophilus, a leading model organism for studies of DNA transporters in thermophilic bacteria, seven competence proteins play a dual role in both systems, whereas two competence genes, comEA and comEC, are suggested to represent unique DNA translocator proteins. Here we show that the T. thermophilus ComEA protein binds dsDNA and is anchored in the inner membrane. comEA is co-transcribed with the flanking comEC gene, and transcription of this operon is upregulated by nutrient limitation and low temperature. To our surprise, a comEC mutant was impaired in piliation. We followed this observation and uncovered that the impaired piliation of the comEC mutant is due to a transcriptional downregulation of pilA4 and the pilN both playing a dual role in piliation and natural competence. Moreover, the comEC mutation resulted in a dramatic decrease in mRNA levels of the pseudopilin gene pilA1, which is unique for the DNA transporter. We conclude that ComEC modulates transcriptional regulation of type IV pili and DNA translocator components thereby mediating a response to extracellular parameters.

Different effects of MglA and MglB on pilus-mediated functions and natural competence in Thermus thermophilus

The thermophilic bacterium Thermus thermophilus is known for its high natural competence. Uptake of DNA is mediated by a DNA translocator that shares components with type IV pili. Localization and function of type IV pili in other bacteria depend on the cellular localization at the poles of the bacterium, a process that involves MglA and MglB. T. thermophilus contains homologs of MglA and MglB. The genes encoding MglA and MglB were deleted and the physiology of the mutants was studied. Deletion of the genes individually or in tandem had no effect on pili formation but pili lost their localization at the poles. The mutants abolished pilus-mediated functions such as twitching motility and adherence but had no effect on uptake of DNA by natural competence. These data demonstrate that MglA and MglB are dispensable for natural transformation and are consistent with the hypothesis that uptake of DNA does not depend on type IV pili or their cellular localization.

Zinc and ATP binding of the hexameric AAA-ATPase PilF from Thermus thermophilus: role in complex stability, piliation, adhesion, twitching motility, and natural transformation

The traffic AAA-ATPase PilF is essential for pilus biogenesis and natural transformation of Thermus thermophilus HB27. Recently, we showed that PilF forms hexameric complexes containing six zinc atoms coordinated by conserved tetracysteine motifs. Here we report that zinc binding is essential for complex stability. However, zinc binding is neither required for pilus biogenesis nor natural transformation. A number of the mutants did not exhibit any pili during growth at 64 °C but still were transformable. This leads to the conclusion that type 4 pili and the DNA translocator are distinct systems. At lower growth temperatures (55 °C) the zinc-depleted multiple cysteine mutants were hyperpiliated but defective in pilus-mediated twitching motility. This provides evidence that zinc binding is essential for the role of PilF in pilus dynamics. Moreover, we found that zinc binding is essential for complex stability but dispensable for ATPase activity. In contrast to many polymerization ATPases from mesophilic bacteria, ATP binding is not required for PilF complex formation; however, it significantly increases complex stability. These data suggest that zinc and ATP binding increase complex stability that is important for functionality of PilF under extreme environmental conditions.

Environmental factors affecting the expression of type IV pilus genes as well as piliation of Thermus thermophilus

The thermophilic bacterium Thermus thermophilus HB27 is known for its highly efficient natural transformation system, which has become a model system to study the structure and function of DNA transporter in thermophilic bacteria. The DNA transporter is functionally linked to type IV pili (T4P), which are essential for twitching motility and adhesion to solid surfaces. However, the pilus structures themselves are dispensable for natural transformation. Here, we report that the cellular mRNA levels of the major structural subunit of the T4P, PilA4, are regulated by environmental factors. Growth of T. thermophilus in minimal medium or low temperature (55 °C) leads to a significant increase in pilA4 transcripts. In contrast, the transcript levels of the minor pilin pilA1 as well as other T4P genes are nearly unaffected. The elevated pilA4 mRNA levels are accompanied by an increase in piliation of the cells but not by elevated natural transformation frequencies. Hyperpiliation leads to increased adhesion to plastic surfaces. The increased cell-surface interactions are suggested to represent an adaptive response to temperature stress and may be advantageous for survival of T. thermophilus.