RESUMO
The Bdellovibrio are miniature "living antibiotic" predatory bacteria which invade, reseal, and digest other larger Gram-negative bacteria, including pathogens. Nutrients for the replication of Bdellovibrio bacteria come entirely from the digestion of the single invaded bacterium, now called a bdelloplast, which is bound by the original prey outer membrane. Bdellovibrio bacteria are efficient digesters of prey cells, yielding on average 4 to 6 progeny from digestion of a single prey cell of a genome size similar to that of the Bdellovibrio cell itself. The developmental intrabacterial cycle of Bdellovibrio is largely unknown and has never been visualized "live." Using the latest motorized xy stage with a very defined z-axis control and engineered periplasmically fluorescent prey allows, for the first time, accurate return and visualization without prey bleaching of developing Bdellovibrio cells using solely the inner resources of a prey cell over several hours. We show that Bdellovibrio bacteria do not follow the familiar pattern of bacterial cell division by binary fission. Instead, they septate synchronously to produce both odd and even numbers of progeny, even when two separate Bdellovibrio cells have invaded and develop within a single prey bacterium, producing two different amounts of progeny. Evolution of this novel septation pattern, allowing odd progeny yields, allows optimal use of the finite prey cell resources to produce maximal replicated, predatory bacteria. When replication is complete, Bdellovibrio cells exit the exhausted prey and are seen leaving via discrete pores rather than by breakdown of the entire outer membrane of the prey.
Assuntos
Bdellovibrio/citologia , Bdellovibrio/fisiologia , Bdellovibrio/ultraestrutura , Divisão Celular/fisiologia , Escherichia coli/citologia , Escherichia coli/genética , Escherichia coli/fisiologia , Proteínas Luminescentes/genética , Microscopia Eletrônica , Microscopia de Fluorescência/métodosRESUMO
Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant length. Here, we examined hook length and found that it was determined not by molecular rulers but probably by the amount of subunit protein secreted by the flagellar export apparatus. The export apparatus shares common features with the type III virulence-factor secretion machinery and thus may be used more widely in length determination of structures other than flagella.
Assuntos
Proteínas de Bactérias/metabolismo , Flagelos/ultraestrutura , Flagelina/metabolismo , Salmonella typhimurium/ultraestrutura , Sítios de Ligação , Flagelos/metabolismo , Flagelos/fisiologia , Genes Bacterianos , Microscopia Eletrônica , Movimento , Mutação , Transporte Proteico , Salmonella typhimurium/genética , Salmonella typhimurium/metabolismo , Salmonella typhimurium/fisiologiaRESUMO
The type III secretion system of Salmonella typhimurium directs the translocation of proteins into host cells. Evolutionarily related to the flagellar assembly machinery, this system is also present in other pathogenic bacteria, but its organization is unknown. Electron microscopy revealed supramolecular structures spanning the inner and outer membranes of flagellated and nonflagellated strains; such structures were not detected in strains carrying null mutations in components of the type III apparatus. Isolated structures were found to contain at least three proteins of this secretion system. Thus, the type III apparatus of S. typhimurium, and presumably other bacteria, exists as a supramolecular structure in the bacterial envelope.
Assuntos
Proteínas de Bactérias/análise , Proteínas de Bactérias/metabolismo , Proteínas de Membrana/análise , Proteínas de Membrana Transportadoras , Salmonella typhimurium/química , Salmonella typhimurium/ultraestrutura , Proteínas da Membrana Bacteriana Externa/análise , Proteínas de Bactérias/química , Proteínas de Bactérias/ultraestrutura , Membrana Celular/química , Membrana Celular/ultraestrutura , Centrifugação com Gradiente de Concentração , Substâncias Macromoleculares , Proteínas de Membrana/química , Proteínas de Membrana/ultraestrutura , Microscopia Eletrônica , Microscopia Imunoeletrônica , Porinas/análise , Salmonella typhimurium/metabolismoRESUMO
Polyhooks were isolated from Salmonella SJW880, a non-flagellated mutant, and purified by cesium chloride density gradient centrifugation. The polyhooks were disintegrated into protein subunits (monomer) by heat in the absence of salt. The monomer was repolymerized in the presence of moderately high concentrations of sodium citrate at neutral pH. Three types of polymer were produced. One type of polymer, produced at room temperature and at citrate concentrations less than 0.3 M, had no regular shape and no definite thickness. Another type of polymer, produced at room temperature and at citrate concentrations greater than 0.4 M, had a straight shape and a similar thickness to that of polyhook but was easily dissociated into monomer in the absence of salt. A third type of polymer was produced at low temperature, independently of the concentration of citrate, and seemed to be a tubular polymer with a thickness similar to that of polyhook but had no helical curvature. However, this type of polymer was shown to have a structure locally the same as that of polyhook by electron microscopic observation, optical diffraction and circular dichroism measurements.
Assuntos
Proteínas de Bactérias , Flagelos/análise , Salmonella/análise , Proteínas de Bactérias/genética , Dicroísmo Circular , Citratos , Microscopia Eletrônica , Mutação , Polímeros , Conformação Proteica , Salmonella/genéticaRESUMO
The terminal regions of Salmonella flagellin are essential for polymerization to form the flagellar filament. It has recently been suggested, on the basis of results from circular dichroism spectroscopy and scanning calorimetry, that these regions are disordered in solution. We report here direct evidence for disorder and mobility in the terminal regions of flagellin using 400 MHz proton nuclear magnetic resonance (n.m.r.) spectroscopy. Comparison of the n.m.r. spectra of monomeric and polymeric flagellin shows that the terminal regions become organized when polymerized to form the filament.
Assuntos
Proteínas de Bactérias , Flagelos/ultraestrutura , Flagelina , Salmonella/ultraestrutura , Proteínas de Bactérias/metabolismo , Flagelina/metabolismo , Substâncias Macromoleculares , Espectroscopia de Ressonância Magnética/métodosRESUMO
Frameshift mutations in the fliK gene of Salmonella result in abnormal elongation of the hook and the failure to assemble filament (polyhook phenotype). Second-site suppressor mutations restore filament assembly, but the cells often remain defective in hook-length control (polyhook-filament phenotype). Where the suppressor mutations are intragenic, the second mutation restores the original frame, generating a region of frameshifted sequence, but restoring the natural C terminus. Some of these frameshifted sequences contain a UGA (opal) termination codon. These cells have few flagella and swarm poorly. We suspected that readthrough of UGA by tRNATrp might be the reason for the partial function. When the UGA codon was changed to the Trp codon UGG, flagellar assembly and function were restored to wild-type levels. Conversely, underexpression of the wild-type fliK gene, achieved by changing the sole Trp codon in the sequence (Trp271) to UGA, decreased both the number of flagella and the ability to swarm. These results validate the readthrough hypothesis and indicate that low levels of FliK sustain some degree of flagellation and motility. At low levels of FliK, most flagella had polyhooks. With increasing amounts, the morphology progressively changed to polyhook-filament, and eventually to wild-type hook-filament. When FliK was overproduced, the hook length was slightly shorter (46(+/-7) nm) than that of the wild-type strain (55(+/-9) nm). FliK levels were measured by immunoblotting. Wild-type levels were about 40 to 80 molecules/cell. FliK synthesized by UGA readthrough could be detected when overproduced from plasmid fliK-W271opal, and the levels indicated a probability of readthrough of 0.002 to 0.01. This value was used to estimate the cellular level of underexpressed FliK, which could partly restore function to a fliK mutant, at about 0.07 to 0.8 molecule/cell. These results suggest that FliK does not form a large structure in the cytoplasm and may function as a regulatory protein for protein export. A model for hook-length control is presented that involves feedback from the assembly point to the export apparatus.
Assuntos
Proteínas de Bactérias/metabolismo , Flagelos/metabolismo , Salmonella typhimurium/crescimento & desenvolvimento , Salmonella typhimurium/metabolismo , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Sequência de Bases , Clonagem Molecular , Códon de Terminação/genética , Flagelos/ultraestrutura , Mutação da Fase de Leitura , Regulação Bacteriana da Expressão Gênica , Genes Bacterianos , Modelos Biológicos , Mutagênese Sítio-Dirigida , Fenótipo , RNA Bacteriano/metabolismo , RNA de Transferência de Triptofano/metabolismo , Salmonella typhimurium/genética , Transdução de Sinais , Supressão GenéticaRESUMO
Certain classes of pathogenic bacteria secrete virulence proteins in a Sec-independent manner, by a mechanism known as type III secretion. The main body of the export apparatus specific for virulence proteins is identified as a needle complex, which has a similar structural organization to flagella. The two structures share several proteins with highly homologous amino acid sequences. Even where the sequence identity is low among flagellar proteins from various species, the physico-chemical properties of each protein remain homologous. Therefore, by comparing the physico-chemical properties of unidentified proteins, it is possible to find homologs among type III secretion systems.
Assuntos
Bactérias/química , Proteínas de Bactérias/química , Flagelos/química , Bactérias/patogenicidade , Proteínas de Bactérias/genética , Análise de SequênciaRESUMO
When motile swarmer cells of Caulobacter crescentus differentiate into sessile stalked cells, the flagellum is ejected. To elucidate the molecular mechanism of the flagellar ejection, flagellar hook-basal body (HBB) complexes from C. crescentus were purified and characterized. The purified HBBs were less stable against acidic pH or protease treatment than HBBs of Salmonella typhimurium, supporting the view that flagellar ejection from C. crescentus is initiated by destruction of the fragile basal structures. In addition, protease treatment of the purified flagella resulted in the specific digestion of the MS ring complex, revealing for the first time the intact structure of the whole rod.
Assuntos
Caulobacter crescentus/crescimento & desenvolvimento , Caulobacter crescentus/fisiologia , Flagelos/fisiologia , Flagelos/ultraestrutura , Peptídeo Hidrolases/metabolismo , Sequência de Aminoácidos , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Caulobacter crescentus/metabolismo , Caulobacter crescentus/ultraestrutura , Centrifugação com Gradiente de Concentração , Regulação Bacteriana da Expressão Gênica , Concentração de Íons de Hidrogênio , Dados de Sequência Molecular , MorfogêneseRESUMO
The bacterial flagellum is a motility apparatus in which a long helical filament--the propeller--is driven by a rotary motor embedded in the cell surface. Out of more than 40 genes required for construction of a fully functional flagellum in Salmonella typhimurium, only 18 gene products have been identified in the mature structure. Some other flagellar proteins play logistical roles during construction, which involves the selective export of flagellar components through a central hole in the flagellum. The whole structure is constructed from base to tip by linear assembly; that is, by adding new components on the growing end, resulting in the distal growth of each substructure. Components of the substructures do not necessarily self-assemble, but often demand the help of other proteins. Recent progress in the understanding of flagellar assembly, which has been most extensively studied in S. typhimurium, is reviewed.
Assuntos
Flagelos/química , Salmonella typhimurium/metabolismo , Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Flagelos/genética , Flagelos/metabolismo , Regulação Bacteriana da Expressão Gênica/genética , Proteínas de Membrana/química , Proteínas de Membrana/metabolismo , Modelos Moleculares , Óperon/genéticaRESUMO
The peritrichous flagella of Salmonella are synthesized and function through many cell generations. There are two different aspects in the relationship between flagellar biogenesis and cell division. Filament growth is independent from the cell cycle and the length of filaments appear to be locally controlled at each flagellar base, whereas the number of filaments (or flagellar basal bodies) is dependent on cell cycle. We present a model to explain how the number of filaments is maintained through generations. We will also introduce a new direction for research that might directly connect flagellation and cell division; the global communication between flagellar genes and external factors of a complex regulatory network in a cell.
Assuntos
Flagelos/fisiologia , Salmonella/crescimento & desenvolvimento , Divisão Celular , Proteínas de Ligação a DNA/genética , Proteínas de Escherichia coli , Flagelos/genética , Flagelos/ultraestrutura , Regulação Bacteriana da Expressão Gênica , Salmonella/genética , Salmonella/ultraestrutura , Transativadores/genéticaRESUMO
CheY is the response regulator of bacterial chemotaxis. Previously, we showed that CheY binds to the flagellar switch protein FliM and that this binding is increased upon phosphorylation of CheY [Welch, M., Oosawa, K., Aizawa, S.-I., & Eisenbach, M. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8787-8791]. Here, we demonstrate that it is the phosphorylated conformation of CheY, rather than the phosphate group itself, that is recognized and bound by FliM. We found that subsequent to the phosphorylation of CheY, Mg2+ was not required for the binding of CheY to FliM. However, phosphorylation of CheY did cause a change in the coordination properties of Mg2+ in the acid pocket of the protein. This change in the coordination of Mg2+ required the presence of the absolutely conserved residue Lys109. When Lys109 was substituted by arginine, the resulting CheY protein was unable to adopt an active conformation upon phosphorylation, and the protein was not bound by FliM. Surprisingly, the CheY13DK mutant protein, which is active in vivo but cannot be phosphorylated in vitro, exhibited only a low level of FliM binding activity, suggesting that its ability to cause clockwise rotation in the cell is not due to a constitutively high level of FliM binding. On the basis of these findings, we propose a mechanism for CheY activation by phosphorylation.
Assuntos
Proteínas de Bactérias/metabolismo , Quimiotaxia/fisiologia , Flagelos/fisiologia , Proteínas de Membrana/metabolismo , Escherichia coli/genética , Escherichia coli/fisiologia , Proteínas de Escherichia coli , Magnésio/metabolismo , Magnésio/farmacologia , Proteínas de Membrana/química , Proteínas de Membrana/genética , Proteínas Quimiotáticas Aceptoras de Metil , Mutagênese Sítio-Dirigida , Fosforilação , Ligação Proteica/efeitos dos fármacos , Conformação Proteica , Espectrometria de FluorescênciaRESUMO
The flaAII gene of Salmonella typhimurium has also been termed motC and cheV, because defective alleles may give rise to a nonflagellate, paralyzed, or nonchemotactic phenotype. We isolated a temperature-sensitive motility mutant (MY1) and have found that the mutation occurs in the flaAII gene. In temperature-jump experiments, MY1 could be converted from highly motile to paralyzed within 0.5 s, demonstrating that flaAII is a structural gene whose product is immediately essential for motor rotation. The mutant, although chemotactic at permissive temperatures (less than 36 degrees C), had a higher clockwise rotational bias than did the wild type; it can therefore be regarded simultaneously as motC(Ts) and cheV (tumbly). The only previously reported S. typhimurium cheV mutant was smooth-swimming. A shift toward counterclockwise bias accompanied loss of rotational speed in the restrictive temperature range. This result, by analogy with known proton motive force effects on motor switching, further indicates a central role of the flaAII (motC, cheV) protein in the energy transduction and switching process. Since there is no evidence associating it with the isolable entity known as the basal body, it may reside at the cytoplasmic face of the flagellar motor.
Assuntos
Flagelos/fisiologia , Genes Bacterianos , Genes , Salmonella typhimurium/genética , Transferência de Energia , Teste de Complementação Genética , Microscopia Eletrônica , Mutação , Salmonella typhimurium/fisiologia , TemperaturaRESUMO
The growth rate of flagellar hooks in Salmonella typhimurium was analyzed by computer-aided simulation of the length distributions of mutant hooks of uncontrolled length (polyhooks). The wild-type hook has a relatively well-controlled length, with an average of 55 nm and a standard deviation of 6 nm. Mutations in the fliK gene give rise to polyhooks. A histogram of the lengths of polyhooks from a fliK mutant shows a peak at 55 nm with a long monotonic tail extending out to 1 microm. To analyze the growth rate, we employed the population balance method. Regression analysis showed that the histogram could fit a combination of two theoretical curves. In the first phase of growth, the hook starts with a very fast growth rate (40 nm/min), and then the rate exponentially slows until the length reaches 55 nm. In the second phase of growth, where the hook length is over 55 nm, the hook grows at a constant rate of 8 nm/min. Second mutations in either the fliK or flhB genes, as found in pseudorevertants from fliK mutants, give rise to polyhook filaments (phf). The ratio between the numbers of hooks with and without filament was 6:4. The calculated probability of filament attachment to polyhooks was low so that the proportion of hooks that start filament growth was only 2% per minute. The lengths of polyhooks with and without filaments were measured. A histogram of hook length in phf's was the same as that for polyhooks in single-site fliK mutants, against the expectation that the distribution would shift to a shorter average. The role of FliK in hook length control is discussed.
Assuntos
Flagelos/fisiologia , Flagelos/ultraestrutura , Salmonella/fisiologia , Proteínas de Bactérias/genética , Proteínas de Bactérias/fisiologia , Divisão Celular , Cinética , Matemática , Microscopia Eletrônica , Modelos Biológicos , Modelos Estatísticos , Densidade Demográfica , Salmonella/citologia , Salmonella/genéticaRESUMO
A Salmonella typhimurium strain possessing a mutation in the fliF gene (coding for the component protein of the M ring of the flagellar basal body) swarmed poorly on a semisolid plate. However, cells grown in liquid medium swam normally and did not show any differences from wild-type cells in terms of swimming speed or tumbling frequency. When mutant cells were grown in a viscous medium, detached bundles of flagellar filaments as long as 100 microns were formed and the cells had impaired motility. Electron microscopy and immunoelectron microscopy revealed that the filaments released from the cells had the hook and a part of the rod of the flagellar basal body still attached. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis showed that the rod portion of the released structures consisted of the 30-kilodalton FlgG protein. Double mutants containing this fliF mutation and various che mutations were constructed, and their behavior in viscous media was analyzed. When the flagellar rotation of the mutants was strongly biased to either a counterclockwise or a clockwise direction, detached bundles were not formed. The formation of large bundles was most extreme in mutants weakly biased to clockwise rotation.
Assuntos
Salmonella typhimurium/genética , Proteínas de Bactérias/análise , Eletroforese em Gel Bidimensional , Flagelos/ultraestrutura , Microscopia Eletrônica , Peso Molecular , Movimento , Mutação , Salmonella typhimurium/ultraestruturaRESUMO
Flagellar motility in Rhodobacter sphaeroides is notably different from that in other bacteria. R. sphaeroides moves in a series of runs and stops produced by the intermittent rotation of the flagellar motor. R. sphaeroides has a single, plain filament whose conformation changes according to flagellar motor activity. Conformations adopted during swimming include coiled, helical, and apparently straight forms. This range of morphological transitions is larger than that in other bacteria, where filaments alternate between left- and right-handed helical forms. The polymorphic ability of isolated R. sphaeroides filaments was tested in vitro by varying pH and ionic strength. The isolated filaments could form open-coiled, straight, normal, or curly conformations. The range of transitions made by the R. sphaeroides filament differs from that reported for Salmonella enterica serovar Typhimurium. The sequence of the R. sphaeroides fliC gene, which encodes the flagellin protein, was determined. The gene appears to be controlled by a sigma(28)-dependent promoter. It encodes a predicted peptide of 493 amino acids. Serovar Typhimurium mutants with altered polymorphic ability usually have amino acid changes at the terminal portions of flagellin or a deletion in the central region. There are no obvious major differences in the central regions to explain the difference in polymorphic ability. In serovar Typhimurium filaments, the termini of flagellin monomers have a coiled-coil conformation. The termini of R. sphaeroides flagellin are predicted to have a lower probability of coiled coils than are those of serovar Typhimurium flagellin. This may be one reason for the differences in polymorphic ability between the two filaments.
Assuntos
Flagelos/fisiologia , Flagelina/genética , Flagelina/metabolismo , Polimorfismo Genético , Rhodobacter sphaeroides/fisiologia , Sequência de Aminoácidos , Proteínas de Bactérias/metabolismo , Sequência de Bases , Sítios de Ligação , Clonagem Molecular , Sequência Consenso , Flagelos/genética , Flagelos/ultraestrutura , Flagelina/química , Genes Bacterianos , Concentração de Íons de Hidrogênio , Dados de Sequência Molecular , Movimento , Proteínas Recombinantes/química , Proteínas Recombinantes/metabolismo , Rhodobacter sphaeroides/genética , Rhodobacter sphaeroides/ultraestrutura , Alinhamento de Sequência , Homologia de Sequência de Aminoácidos , Fator sigma/metabolismoRESUMO
Pseudorevertants (second-site suppressor mutants) were isolated from a set of parental mutants of Salmonella with defects in the flagellar switch genes fliG and fliM. Most of the suppressing mutations lay in flagellar region IIIb of the chromosome. One fliG mutant, SJW2811, gave rise to a large number of suppressor mutations in the motility genes motA and motB, which are in flagellar region II. SJW2811, which has a three-amino-acid deletion (delta Pro-Ala-Ala) at positions 169 to 171 of FliG, had an extreme clockwise motor bias that produced inverse smooth swimming (i.e., swimming by means of clockwise rotation of a hydrodynamically induced right-handed helical bundle), and formed Mot(-)-like colonies on semisolid medium. Unlike previously reported inverse-swimming mutants, it did not show a chemotactic response to serine, and it remained inverse even in a delta che background; thus, its switch is locked in the clockwise state. The location of the mutation further underscores the conclusion from a previous study of spontaneous missense mutants (V. M. Irikura, M. Kihara, S. Yamaguchi, H. Sockett, and R. M. Macnab, J. Bacteriol. 175:802-810, 1993) that a relatively localized region in the central part of the FliG sequence is critically important for switching. All of the second-site mutations in motA and motB caused some impairment of motility, both in the pseudorevertants and in a wild-type fliG background. The mechanism of suppression of the fliG mutation by the mot mutations is complex, involving destabilization of the right-handed flagellar bundle as a result of reduced motor speed. The mutations in the MotA and MotB sequences were clustered to a considerable degree as follows: in transmembrane helices 3 and 4 of MotA and the sole transmembrane helix of MotB, at helix-membrane interfaces, in the cytoplasmic domains of MotA, and in the vicinity of the peptidoglycan binding region of the periplasmic domain of MotB. The potential importance of Lys28 and Asp33 of the MotB sequence for proton delivery to the site of torque generation is discussed.
Assuntos
Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Genes Bacterianos , Mutação Puntual , Estrutura Secundária de Proteína , Salmonella typhimurium/fisiologia , Supressão Genética , Sequência de Aminoácidos , Proteínas de Bactérias/biossíntese , Movimento Celular , Genótipo , Modelos Estruturais , Dados de Sequência Molecular , Salmonella typhimurium/genética , Deleção de Sequência , Homologia de Sequência de AminoácidosRESUMO
The flagellar hook-basal body (HBB) complex of the Gram-positive bacterium Bacillus subtilis was purified and analysed by electron microscopy, gel electrophoresis, and amino acid sequencing of the major component proteins. The purified HBB complex consisted of the inner (M and S) rings, a rod and a hook. There were no outer (P and L) rings that are found in Gram-negative bacteria. The hook was 15 nm in thickness and 70 nm in length, which is thinner and longer than the hook of Salmonella typhimurium. The hook protein had an apparent molecular mass of 29 kDa, and its N-terminal sequence was identical to that of B. subtilis FIgG, which was previously reported as a rod protein. The sequence of the reported FIgG protein of B. subtilis is more closely related to that of FIgE (the hook protein) rather than FIgG (the rod protein) of S. typhimurium, in spite of the difference of the apparent molecular masses between the two hook proteins (29 kDa versus 42 kDa). The hook-basal body contained six major proteins (with apparent molecular masses of 82, 59, 35, 32, 29 and 20 kDa) and two minor proteins (23 kDa and 13 kDa), which consistently appeared from preparation to preparation. The N-terminus of each of these proteins was sequenced. Comparison with protein databases revealed the following polypeptide-gene correspondences: 82 kDa, fIiF; 59 kDa, fIgK; 35 kDa, orfF; 32 kDa, yqhF; 23 kDa, orf3 of the fIaA locus; 20 kDa, fIgB and fIgC; 13 kDa, not determined. The band at 20 kDa was a mixture of FIgB and FIgC, as revealed by two-dimensional gel analysis. Characteristic features of B. subtilis HBB are discussed in comparison with those of S. typhimiurium.
Assuntos
Bacillus subtilis/química , Flagelos/química , Sequência de Aminoácidos , Proteínas de Bactérias/análise , Proteínas de Bactérias/isolamento & purificação , Flagelos/ultraestrutura , Microscopia Eletrônica , Dados de Sequência Molecular , Salmonella typhimurium/química , Análise de Sequência , Homologia de Sequência de AminoácidosRESUMO
Many bacterial pathogens of plants and animals have evolved a specialized protein-secretion system termed type III to deliver bacterial proteins into host cells. These proteins stimulate or interfere with host cellular functions for the pathogen's benefit. The Salmonella typhimurium pathogenicity island 1 encodes one of these systems that mediates this bacterium's ability to enter nonphagocytic cells. Several components of this type III secretion system are organized in a supramolecular structure termed the needle complex. This structure is made of discrete substructures including a base that spans both membranes and a needle-like projection that extends outward from the bacterial surface. We demonstrate here that the type III secretion export apparatus is required for the assembly of the needle substructure but is dispensable for the assembly of the base. We show that the length of the needle segment is determined by the type III secretion associated protein InvJ. We report that InvG, PrgH, and PrgK constitute the base and that PrgI is the main component of the needle of the type III secretion complex. PrgI homologs are present in type III secretion systems from bacteria pathogenic for animals but are absent from bacteria pathogenic for plants. We hypothesize that the needle component may establish the specificity of type III secretion systems in delivering proteins into either plant or animal cells.