Two DNA-binding proteins with similarity to eukaryotic histone H1 Chlamydia trachomatis is an obligate intracellular parasitic bacterium with a biphasic developmental cycle alternating between the extracellular, metabolically inactive elementary body (EB) form and the intracellular metabolically active reticulate body (RB) form that replicates within the eukaryotic host cell (24). The chlamydial cycle of infection begins with the uptake of an EB by a susceptible host cell into a membrane-bound vacuole. Within approximately 8 h, differentiation to the larger RB form begins, marked by the reduction of the disulfide-linked EB outer membrane complex and dispersal of the highly condensed EB nucleoid. RBs multiply by binary fission within the cellular inclusion until 24 to 36 h after their internalization, at which time the developmental transition from RB back to EB begins. Characteristic compaction of the nucleoid occurs, accompanied by oxidative disulfide cross-linking of the outer membrane complex to form the mechanically rigid, impermeable EB cell wall.

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Chlamydiae are obligate intracellular bacteria which undergo a unique developmental cycle, alternating between non-replicative elementary bodies (EBs) and replicative reticulate bodies (RBs). The transition from RB to EB is characterized by condensation of the chromosome into a dense nucleoid structure. The chlamydial histone homologue Hc1 is sufficient to induce formation of a similar structure in Escherichia coli. High-level Hc1 expression in E. coli is self-limiting and down-regulates transcription, translation, and replication at concentrations similar to those observed in chlamydial elementary bodies. Expression of Hc1 at sub-structural levels may have specific regulatory functions through its interaction with chromosomal DNA. In E. coli this is reflected in a dramatic shift in the pattern of gene expression. The differential expression of the outer membrane porin proteins OmpC and OmpF and analysis of lacZ fusions with promoter regions sensitive to supercoiling suggests that low-level Hc1 expression results in a net relaxation of chromosomal DNA. Topological analysis of plasmid DNA from both E. coli and Chlamydia trachomatis supports a decrease in superhelicity preceding nucleoid formation. In vitro analysis of purified Hc1-DNA interactions supports preferential binding based upon DNA conformation. These results suggest a dual role in which Hc1-mediated changes in gene expression may precede metabolic inactivity.

Bordetella bronchiseptica mutants BRM1, BRM6, and BRM9 fail to produce the native dihydroxamate siderophore alcaligin. A 4.5-kb BamHI-SmaI Bordetella pertussis genomic DNA fragment carried multiple genes required to restore alcaligin production to these siderophore-deficient mutants. Phenotypic complementation analysis using subclones of the 4.5-kb genomic region demonstrated that the closely linked BRM1 and BRM9 mutations were genetically separable from the BRM6 mutation, and both insertions exerted strong polar effects on expression of the downstream gene defined by the BRM6 mutation, suggesting a polycistronic transcriptional organization of these alcaligin biosynthesis genes. Subcloning and complementation experiments localized the putative Bordetella promoter to a 0.7-kb BamHI-SphI subregion of the cloned genomic DNA fragment. Nucleotide sequencing, phenotypic analysis of mutants, and protein expression by the 4.5-kb DNA fragment in Escherichia coli suggested the presence of three alcaligin system genes, namely, alcA, alcB, and alcC. The deduced protein products of alcA, alcB, and alcC have significant primary amino acid sequence similarities with known microbial siderophore biosynthesis enzymes. Primer extension analysis mapped the transcriptional start site of the putative alcaligin biosynthesis operon containing alcABC to a promoter region overlapping a proposed Fur repressor-binding site and demonstrated iron regulation at the transcriptional level.Iron is a fundamental nutritional requirement for virtually all cells, and its assimilation is considered essential for invading pathogenic bacteria to establish infection in the iron-limiting environment of the host (13, 56). Additionally, iron serves as an environmental modulator of the production of certain virulence factors in a number of bacteria (14,24,31,32,46). Despite host iron sequestration, mediated primarily by the glycoprotein family of iron-binding transferrins, pathogens multiply successfully in vivo because they express efficient ironscavenging systems in response to decreased iron availability. These iron retrieval systems utilize two general strategies: one involving high-affinity iron-chelating soluble siderophores (30,40) and the other using siderophore-independent cell surface receptor mechanisms allowing iron uptake directly from host sources such as transferrin, lactoferrin, and heme compounds (7,35,38,54).Bordetella pertussis, the causative agent of human whooping cough or pertussis, and Bordetella bronchiseptica, the agent of swine atrophic rhinitis and kennel cough in dogs, are bacterial pathogens that infect the respiratory epithelial mucosae of their hosts. Early reports described the production of putative siderophores by both B. pertussis and B. bronchiseptica in response to iron deficiency (1, 23). Armstrong and Clements isolated and characterized B. bronchiseptica transposon-induced siderophore-deficient mutants; DNA hybridization studies using sequences flanking those transposon insertions confirmed the existence of homologs o...

The redox behavior of Fe(III) complexes of the cyclic hydroxamate siderophores alcaligin and desferrioxamine E was investigated by cyclic voltammetry. The limiting, pH independent redox potential (E(1/2) vs NHE) is -446 mV for alcaligin above pH 9 and -477 mV for ferrioxamine E above pH 7.5. At lower pH values, the redox potential for both complexes shifts positive, with a loss of voltammetric reversibility which is interpreted to be the consequence of a secondary dissociation of Fe(II) from the reduced form of the complexes. These observations are of biological importance, since they suggest the possibility of a reductive mechanism in microbial cells which utilize these siderophores to acquire Fe. For comparison purposes, cyclic voltammograms were obtained for Fe(III) complexes with trihydroxamic acids of cyclic (ferrioxamine E) and linear (ferrioxamine B) structures, with dihydroxamic acids of cyclic (alcaligin) and linear (rhodotorulic and sebacic acids) structures, and with monohydroxamic acids (acetohydroxamic and N-methylacetohydroxamic acids) at identical conditions. The observed redox potentials allow us to estimate the overall stability constants for fully coordinated Fe(II) complexes as log beta(II)(Fe(2)alcaligin(3)) = 24.6 and log beta(II)(ferrioxamine E) = 12.1. A linear correlation between E(1/2) and pM was found, and the basis for this relationship is discussed in terms of structural (denticity and cyclic/acyclic) and electronic differences among the {alkyl-NOH-CO-alkyl} type of hydroxamic acid ligands studied.

Serological studies of patients with pertussis and the identification of antigenic Bordetella pertussis proteins support the hypothesis that B. pertussis perceives an iron starvation cue and expresses multiple iron source utilization systems in its natural human host environment. Furthermore, previous studies using a murine respiratory tract infection model showed that several of these B. pertussis iron systems are required for colonization and persistence and are differentially expressed over the course of infection. The present study examined genome-wide changes in B. pertussis gene transcript abundance in response to iron starvation in vitro. In addition to known iron source utilization genes, we identified a previously uncharacterized iron-repressed cytoplasmic membrane transporter system, fbpABC, that is required for the utilization of multiple structurally distinct siderophores including alcaligin, enterobactin, ferrichrome, and desferrioxamine B. Expression of type III secretion system genes was also found to be upregulated during iron starvation in both B. pertussis strain Tohama I and Bordetella bronchiseptica strain RB50. In a survey of type III secretion system protein production by an assortment of B. pertussis laboratory-adapted and low-passage clinical isolate strains, iron limitation increased the production and secretion of the type III secretion system-specific translocation apparatus tip protein Bsp22 in all Bvg-proficient strains. These results indicate that iron starvation in the infected host is an important environmental cue influencing not only Bordetella iron transport gene expression but also the expression of other important virulence-associated genes.

Biochemical analysis of the enzymatic activity catalyzing the conversion of chorismate to isochorismate in the enterobactin biosynthetic pathway attributed the reaction to the isochorismate synthetase enzyme, designated EntC. However, the lack of mutations defining this activity has hampered the precise identification of the entC structural gene. In this study, we engineered a stable insertion mutation into the chromosomal region between the enterobactin genes fepB and entE. This mutation disrupted the structural gene for a previously identified 44-kilodalton protein and eliminated production of 2,3-dihydroxybenzoic acid, the catechol precursor of enterobactin. The complete nucleotide sequence of this gene was determined and compared with the sequences of other genes encoding chorismate-utilizing proteins. The similarities observed in these comparisons not only indicated that the locus is entC but also supported the premise that these enzymes constitute a family of related proteins sharing a common evolutionary origin. In addition, in this and the accompanying paper (M. S. Nahlik, T. J. Brickman, B. A. Ozenberger, and M. A. McIntosh, J. Bacteriol. 171:784-790, 1989), evidence is presented indicating that the entA product is potentially a secondary factor in the chorismate-toisochorismate conversion and that the prototypic entC lesion (entC401) resides in the structural gene for the EntA protein. Finally, polarity effects from the insertion mutation in entC on downstream biosynthetic genes indicated that this locus is the promoter-proximal cistron in an ent operon comprising at least five genes. Appropriate regulatory signals upstream of entC suggest that this operon is regulated by iron through interaction with the Fur repressor protein.Biosynthesis of the catechol siderophore enterobactin in Escherichia coli is a two-stage process involving the initial production of 2,3-dihydroxybenzoic acid (DHBA) from the aromatic amino acid precursor chorismate and the subsequent conversion of DHBA and L-serine to the active chelator. The three soluble enzymes required for the initial stage are EntC, (isochorismate synthetase), EntB (2,3-dihydro-2,3-dihydroxybenzoate synthetase), and EntA (2,3-dihydro-2,3-dihydroxybenzoate dehydrogenase). Intermediates in the biosynthetic reactions catalyzed by these enzymes have been identified (32). The genes encoding the initialstage enzymes were predicted to be linked transcriptionally on the clockwise end of the complex enterobactin gene cluster at min 13 on the E. coli chromosomal map (7). In the second stage of enterobactin biosynthesis, the proposed enterobactin synthetase multienzyme complex, suggested to contain the entD, entE, entF, and entG gene products (11), catalyzes the synthesis of one enterobactin molecule from three molecules each of DHBA and L-serine. These stage two genes are scattered throughout the enterobactin gene cluster (8,19,24,25).The enterobactin biosynthetic intermediate chorismate also serves as the pivotal substrate for several enzymes associated with key met...

Genetic and biochemical studies have established that Fur and iron mediate repression of Bordetella alcaligin siderophore system (alc) genes under iron-replete nutritional growth conditions. In this study, transcriptional analyses using Bordetella chromosomal alc-lacZ operon fusions determined that maximal alc gene transcriptional activity under iron starvation stress conditions is dependent on the presence of alcaligin siderophore. Mutational analysis and genetic complementation confirmed that alcaligin-responsive transcriptional activation of Bordetella alcaligin system genes is dependent on AlcR, a Fur-regulated AraC-like positive transcriptional regulator encoded within the alcaligin gene cluster. AlcR-mediated transcriptional activation is remarkably sensitive to inducer, occurring at extremely low alcaligin concentrations. This positive autogenous control circuit involving alcaligin siderophore as the inducer for AlcR-mediated transcriptional activation of alcaligin siderophore biosynthesis and transport genes coordinates environmental and intracellular signals for maximal expression of these genes under conditions in which the presence of alcaligin in the environment is perceived.