IST1 insertional inactivation of the resB gene: implications for phenotypic switching in Thiobacillus ferrooxidans

Influence of mobile genetic elements and insertion sequences in long- and short-term adaptive processes of Acidithiobacillus ferrooxidans strains

The recent revision of the Acidithiobacillia class using genomic taxonomy methods has shown that, in addition to the existence of previously unrecognized genera and species, some species of the class harbor levels of divergence that are congruent with ongoing differentiation processes. In this study, we have performed a subspecies-level analysis of sequenced strains of Acidithiobacillus ferrooxidans to prove the existence of distinct sublineages and identify the discriminant genomic/genetic characteristics linked to these sublineages, and to shed light on the processes driving such differentiation. Differences in the genomic relatedness metrics, levels of synteny, gene content, and both integrated and episomal mobile genetic elements (MGE) repertoires support the existence of two subspecies-level taxa within A. ferrooxidans. While sublineage 2A harbors a small plasmid related to pTF5, this episomal MGE is absent in sublineage 2B strains. Likewise, clear differences in the occurrence, coverage and conservation of integrated MGEs are apparent between sublineages. Differential MGE-associated gene cargo pertained to the functional categories of energy metabolism, ion transport, cell surface modification, and defense mechanisms. Inferred functional differences have the potential to impact long-term adaptive processes and may underpin the basis of the subspecies-level differentiation uncovered within A. ferrooxidans. Genome resequencing of iron- and sulfur-adapted cultures of a selected 2A sublineage strain (CCM 4253) showed that both episomal and large integrated MGEs are conserved over twenty generations in either growth condition. In turn, active insertion sequences profoundly impact short-term adaptive processes. The ISAfe1 element was found to be highly active in sublineage 2A strain CCM 4253. Phenotypic mutations caused by the transposition of ISAfe1 into the pstC2 encoding phosphate-transport system permease protein were detected in sulfur-adapted cultures and shown to impair growth on ferrous iron upon the switch of electron donor. The phenotypic manifestation of the △pstC2 mutation, such as a loss of the ability to oxidize ferrous iron, is likely related to the inability of the mutant to secure the phosphorous availability for electron transport-linked phosphorylation coupled to iron oxidation. Depletion of the transpositional △pstC2 mutation occurred concomitantly with a shortening of the iron-oxidation lag phase at later transfers on a ferrous iron-containing medium. Therefore, the pstII operon appears to play an essential role in A. ferrooxidans when cells oxidize ferrous iron. Results highlight the influence of insertion sequences and both integrated and episomal mobile genetic elements in the short- and long-term adaptive processes of A. ferrooxidans strains under changing growth conditions.

Transposase-Mediated Chromosomal Integration of Exogenous Genes in Acidithiobacillus ferrooxidans

The development of Acidithiobacillus ferrooxidans as a non-model host organism for synthetic biology is hampered by a lack of genetic tools and techniques. New plating and liquid-based selection methods were developed to improve the identification of transformed cell lines. Enabled by these methods, a hyperactive transposase was used to generate mutants with integrated genes for the expression of the superfolder green fluorescent protein (sfGFP) gene or a 2-keto decarboxylase (KDC) gene, which enabled the production and secretion of isobutyric acid (IBA). An inverse PCR method was used to identify the insertion sites of the KDC gene in several mutants, leading to the identification of a region on the chromosome that may be suitable for future genetic insertions. These results demonstrate that functional exogenous metabolic genes have been chromosomally integrated into A. ferrooxidans, and this advance will facilitate the future development of these cells for new biotechnology applications.IMPORTANCE Acidithiobacillus ferrooxidans is an iron- and sulfur-oxidizing chemolithoautotroph and is a key member of the microbial consortia used in industrial biomining applications. There is interest in exploiting these cells for other metal recovery applications as well as in developing them as unique nonmodel microbial cell factories. Plasmid-driven expression of exogenous genes has been reported, and homologous recombination has been used to knock out some gene expression. Here, new selection protocols facilitated the development of a transposition method for chromosomal integration of exogenous genes into A. ferrooxidans This greatly expands the available genetic toolbox, which will open the door to greater metabolic engineering efforts for these cells.

Salt Stress-Induced Loss of Iron Oxidoreduction Activities and Reacquisition of That Phenotype Depend on rus Operon Transcription in Acidithiobacillus ferridurans

The type strain of the mineral-oxidizing acidophilic bacterium Acidithiobacillus ferridurans was grown in liquid medium containing elevated concentrations of sodium chloride with hydrogen as electron donor. While it became more tolerant to chloride, after about 1 year, the salt-stressed acidophile was found to have lost its ability to oxidize iron, though not sulfur or hydrogen. Detailed molecular examination revealed that this was due to an insertion sequence, ISAfd1, which belongs to the ISPepr1 subgroup of the IS4 family, having been inserted downstream of the two promoters PI and PII of the rus operon (which codes for the iron oxidation pathway in this acidophile), thereby preventing its transcription. The ability to oxidize iron was regained on protracted incubation of the culture inoculated onto salt-free solid medium containing ferrous iron and incubated under hydrogen. Two revertant strains were obtained. In one, the insertion sequence ISAfd1 had been excised, leaving an 11-bp signature, while in the other an ∼2,500-bp insertion sequence (belonging to the IS66 family) was detected in the downstream inverted repeat of ISAfd1 The transcriptional start site of the rus operon in the second revertant strain was downstream of the two ISs, due to the creation of a new "hybrid" promoter. The loss and subsequent regaining of the ability of A. ferridurans^T to reduce ferric iron were concurrent with those observed for ferrous iron oxidation, suggesting that these two traits are closely linked in this acidophile.IMPORTANCE Iron-oxidizing acidophilic bacteria have primary roles in the oxidative dissolution of sulfide minerals, a process that underpins commercial mineral-processing biotechnologies ("biomining"). Most of these prokaryotes have relatively low tolerance to chloride, which limits their activities when only saline or brackish waters are available. The study showed that it was possible to adapt a typical iron-oxidizing acidophile to grow in the presence of salt concentrations similar to those in seawater, but in so doing they lost their ability to oxidize iron, though not sulfur or hydrogen. The bacterium regained its capacity for oxidizing iron when the salt stress was removed but simultaneously reverted to tolerating lower concentrations of salt. These results suggest that the bacteria that have the main roles in biomining operations could survive but become ineffective in cases where saline or brackish waters are used for irrigation.

The impact of insertion sequences on bacterial genome plasticity and adaptability

Transposable elements (TE), small mobile genetic elements unable to exist independently of the host genome, were initially believed to be exclusively deleterious genomic parasites. However, it is now clear that they play an important role as bacterial mutagenic agents, enabling the host to adapt to new environmental challenges and to colonize new niches. This review focuses on the impact of insertion sequences (IS), arguably the smallest TE, on bacterial genome plasticity and concomitant adaptability of phenotypic traits, including resistance to antibacterial agents, virulence, pathogenicity and catabolism. The direct consequence of IS transposition is the insertion of one DNA sequence into another. This event can result in gene inactivation as well as in modulation of neighbouring gene expression. The latter is usually mediated by de-repression or by the introduction of a complete or partial promoter located within the element. Furthermore, transcription and transposition of IS are affected by host factors and in some cases by environmental signals offering the host an adaptive strategy and promoting genetic variability to withstand the environmental challenges.

Transposition of IS elements induced by electroporation of suicide plasmid in Acidithiobacillus caldus

Transposition insertional mutagenesis of the insertion sequences (IS elements) was discovered for the first time in Acidithiobacillus caldus (A. caldus), when A. caldus MTH-04 hsdM (type I restriction-modification system M-subunit) mutant was constructed by electroporation of a suicide plasmid. The IS element, specifically inserting into hsdM gene, was analyzed, identified, and named ISAtc2. The transposition frequency of ISAtc2 was ranged from 4% to 7%, and no reverse mutation occurred in the mutants after 50 generations of proliferation without selective pressure. These results revealed that transposition of IS elements on A. caldus chromosome could regulate the gene expression and metabolic pathways by gene inactivation, gene loss and gene acquisition. Therefore, the transposition of IS elements in A. caldus may be an important and unique regulation mechanism for adaptation to the living condition.

Acidithiobacillus caldus sulfur oxidation model based on transcriptome analysis between the wild type and sulfur oxygenase reductase defective mutant

BACKGROUND

Acidithiobacillus caldus (A. caldus) is widely used in bio-leaching. It gains energy and electrons from oxidation of elemental sulfur and reduced inorganic sulfur compounds (RISCs) for carbon dioxide fixation and growth. Genomic analyses suggest that its sulfur oxidation system involves a truncated sulfur oxidation (Sox) system (omitting SoxCD), non-Sox sulfur oxidation system similar to the sulfur oxidation in A. ferrooxidans, and sulfur oxygenase reductase (SOR). The complexity of the sulfur oxidation system of A. caldus generates a big obstacle on the research of its sulfur oxidation mechanism. However, the development of genetic manipulation method for A. caldus in recent years provides powerful tools for constructing genetic mutants to study the sulfur oxidation system.

RESULTS

An A. caldus mutant lacking the sulfur oxygenase reductase gene (sor) was created and its growth abilities were measured in media using elemental sulfur (S(0)) and tetrathionate (K(2)S(4)O(6)) as the substrates, respectively. Then, comparative transcriptome analysis (microarrays and real-time quantitative PCR) of the wild type and the Δsor mutant in S(0) and K(2)S(4)O(6) media were employed to detect the differentially expressed genes involved in sulfur oxidation. SOR was concluded to oxidize the cytoplasmic elemental sulfur, but could not couple the sulfur oxidation with the electron transfer chain or substrate-level phosphorylation. Other elemental sulfur oxidation pathways including sulfur diooxygenase (SDO) and heterodisulfide reductase (HDR), the truncated Sox pathway, and the S(4)I pathway for hydrolysis of tetrathionate and oxidation of thiosulfate in A. caldus are proposed according to expression patterns of sulfur oxidation genes and growth abilities of the wild type and the mutant in different substrates media.

CONCLUSION

An integrated sulfur oxidation model with various sulfur oxidation pathways of A. caldus is proposed and the features of this model are summarized.

Comparative genomic analysis of carbon and nitrogen assimilation mechanisms in three indigenous bioleaching bacteria: predictions and validations

BACKGROUND

Carbon and nitrogen fixation are essential pathways for autotrophic bacteria living in extreme environments. These bacteria can use carbon dioxide directly from the air as their sole carbon source and can use different sources of nitrogen such as ammonia, nitrate, nitrite, or even nitrogen from the air. To have a better understanding of how these processes occur and to determine how we can make them more efficient, a comparative genomic analysis of three bioleaching bacteria isolated from mine sites in Chile was performed. This study demonstrated that there are important differences in the carbon dioxide and nitrogen fixation mechanisms among bioleaching bacteria that coexist in mining environments.

RESULTS

In this study, we probed that both Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans incorporate CO2 via the Calvin-Benson-Bassham cycle; however, the former bacterium has two copies of the Rubisco type I gene whereas the latter has only one copy. In contrast, we demonstrated that Leptospirillum ferriphilum utilizes the reductive tricarboxylic acid cycle for carbon fixation. Although all the species analyzed in our study can incorporate ammonia by an ammonia transporter, we demonstrated that Acidithiobacillus thiooxidans could also assimilate nitrate and nitrite but only Acidithiobacillus ferrooxidans could fix nitrogen directly from the air.

CONCLUSION

The current study utilized genomic and molecular evidence to verify carbon and nitrogen fixation mechanisms for three bioleaching bacteria and provided an analysis of the potential regulatory pathways and functional networks that control carbon and nitrogen fixation in these microorganisms.

Adaptive responses of chemolithoautotrophic acidophilic Acidithiobacillus ferrooxidans to sewage sludge

AIM

The aim of the present study was to investigate the phenotypic and genotypic variability of two strains of Acidithiobacillus ferrooxidans genus during growth in sewage sludge.

METHODS AND RESULTS

Compared with A. ferrooxidans cells grown in mineral medium, those grown in sewage sludge demonstrated remarkable changes in ultrastructure (transmission electron microscopy) and significantly elongated lag phases. These latter cells also lacked carboxysomes and rusticyanin, showed lower level of cytochromes and exhibited modifications to their outer membrane proteins (SDS-PAGE). Restriction fragment length polymorphism analysis using pulsed-field gel electrophoresis showed that most restriction fragments were highly conserved and shared by strains grown under different conditions. However, in relation to cells grown in mineral medium, sludge-grown A. ferrooxidans lacked a number of restriction fragments, clearly indicating structural changes to the chromosomal DNA of the organism.

CONCLUSIONS

In combination, the results of this study provide evidence of adaptive responses by chemolithoautotrophic acidophilic A. ferrooxidans to facilitate growth in sewage sludge.

SIGNIFICANCE AND IMPACT OF THE STUDY

The obtained results are important from scientific as well as industrial application point of view, because they confirmed that A. ferrooxidans highly sensitive to organic compounds bacteria is useful in biotechnologies of heavy metal removal from shale ore, polluted soils and sewage sludge containing organic hazardous compounds.

Differential expression of two bc1 complexes in the strict acidophilic chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans suggests a model for their respective roles in iron or sulfur oxidation

Three strains of the strict acidophilic chemolithoautotrophic Acidithiobacillus ferrooxidans, including the type strain ATCC 23270, contain a petIIABC gene cluster that encodes the three proteins, cytochrome c1, cytochrome b and a Rieske protein, that constitute a bc1 electron-transfer complex. RT-PCR and Northern blotting show that the petIIABC cluster is co-transcribed with cycA, encoding a cytochrome c belonging to the c4 family, sdrA, encoding a putative short-chain dehydrogenase, and hip, encoding a high potential iron-sulfur protein, suggesting that the six genes constitute an operon, termed the petII operon. Previous results indicated that A. ferrooxidans contains a second pet operon, termed the petI operon, which contains a gene cluster that is similarly organized except that it lacks hip. Real-time PCR and Northern blot experiments demonstrate that petI is transcribed mainly in cells grown in medium containing iron, whereas petII is transcribed in cells grown in media containing sulfur or iron. Primer extension experiments revealed possible transcription initiation sites for the petI and petII operons. A model is presented in which petI is proposed to encode the bc1 complex, functioning in the uphill flow of electrons from iron to NAD(P), whereas petII is suggested to be involved in electron transfer from sulfur (or formate) to oxygen (or ferric iron). A. ferrooxidans is the only organism, to date, to exhibit two functional bc1 complexes.

Apparent redundancy of electron transfer pathways via bc(1) complexes and terminal oxidases in the extremophilic chemolithoautotrophic Acidithiobacillus ferrooxidans

Acidithiobacillus ferrooxidans is an acidophilic chemolithoautotrophic bacterium that can grow in the presence of either the weak reductant Fe(2+), or reducing sulfur compounds that provide more energy for growth than Fe(2+). We have previously shown that the uphill electron transfer pathway between Fe(2+) and NAD(+) involved a bc(1) complex that functions only in the reverse direction [J. Bacteriol. 182, (2000) 3602]. In the present work, we demonstrate both the existence of a bc(1) complex functioning in the forward direction, expressed when the cells are grown on sulfur, and the presence of two terminal oxidases, a bd and a ba(3) type oxidase expressed more in sulfur than in iron-grown cells, besides the cytochrome aa(3) that was found to be expressed only in iron-grown cells. Sulfur-grown cells exhibit a branching point for electron flow at the level of the quinol pool leading on the one hand to a bd type oxidase, and on the other hand to a bc(1)-->ba(3) pathway. We have also demonstrated the presence in the genome of transcriptionally active genes potentially encoding the subunits of a bo(3) type oxidase. A scheme for the electron transfer chains has been established that shows the existence of multiple respiratory routes to a single electron acceptor O(2). Possible reasons for these apparently redundant pathways are discussed.

Biochemistry, regulation and genomics of haem biosynthesis in prokaryotes

Haems are involved in many cellular processes in prokaryotes and eukaryotes. The biosynthetic pathway leading to haem formation is, with few exceptions, well-conserved, and is controlled in accordance with cellular function. Here, we review the biosynthesis of haem and its regulation in prokaryotes. In addition, we focus on a modification of haem for cytochrome c biogenesis, a complex process that entails both transport between cellular compartments and a specific thioether linkage between the haem moiety and the apoprotein. Finally, a whole genome analysis from 63 prokaryotes indicates intriguing exceptions to the universality of the haem biosynthetic pathway and helps define new frontiers for future study.

Cytochromes c of Acidithiobacillus ferrooxidans

The chemolithoautotrophic Gram-negative bacterium Acidithiobacillus ferrooxidans is versatile and can grow on a number of electron donors and acceptors. In the A. ferrooxidans ATCC 23270 genome, computer analysis identified 11 genes encoding putative cytochromes c. At least eight putative cytochromes c were differentiated on gels in ATCC 33020 cells grown on ferrous iron or sulfur. All these cytochromes were associated with the inner or the outer membranes. Lower levels of total cytochromes c were observed in sulfur- than in ferrous iron-grown cells. One cytochrome c was specific for sulfur conditions while three were specific for iron conditions, suggesting that cytochrome c synthesis is modulated depending on the electron donor.

Characterization of the petI and res operons of Acidithiobacillus ferrooxidans

DNA sequence analysis and bioinformatic interpretations have identified two adjacent clusters of genes potentially involved in the formation of a bc1 complex and in the maturation of a cytochrome c-type protein in two strains (ATCC 19859 and ATCC 33020) of the acidophilic, chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans). Reverse transcriptase-PCR experiments suggest that the two clusters are organized as operons, and +1 start sites of transcription for the operons have been determined by primer extension experiments. Potential promoters have been identified. The presence of these operons lends support to a recent model of reverse electron flow and is consistent with previous reports of phenotypic switching in this bacterium.

ISAfe1, an ISL3 family insertion sequence from Acidithiobacillus ferrooxidans ATCC 19859

A 1.3-kb insertion sequence, termed ISAfe1 (U66426), from Acidithiobacillus ferrooxidans ATCC 19859 is described. ISAfe1 exhibits the features of a typical bacterial insertion sequence. It has 26-bp, imperfectly matched, terminal inverted repeats and an open reading frame (ORF) that potentially encodes a transposase (TPase) of 404 amino acids (AAB07489) with significant similarity to members of the ISL3 family of insertion sequences. A potential ribosome-binding site and potential -10 and -35 promoter sites for the TPase ORF were identified, and a +1 transcriptional start site was detected experimentally. A potential outwardly directed -35 site was identified in the right inverted repeat of ISAfe1. A second ORF (ORF B), of unknown function, was found on the complementary strand with significant similarity to ORF 2 of ISAe1 from Ralstonia eutropha. Southern blot analyses demonstrated that ISAfe1-like elements can be found in multiple copies in a variety of A. ferrooxidans strains and that they exhibit transposition. A codon adaptation index (CAI) analysis of the TPase of ISAfe1 indicates that is has a CAI of 0.726 and can be considered well adapted to its host, suggesting that ISAfe1 might be an ancient resident of A. ferrooxidans. Analysis of six of its target sites of insertion in the genome of A. ferrooxidans ATCC 19859 indicates a preference for 8-bp pseudopalindromic sequences, one of which resembles the termini of its inverted repeats. Evidence is presented here that is consistent with the possibility that ISAfe1 can promote both plasmid cointegrate formation and resolution in E. coli.