Alanyl-tRNA synthetase gene of the extreme acidophilic chemolithoautotrophic Thiobacillus ferrooxidans is highly homologous to alaS genes from all living kingdoms but cannot be transcribed from its promoter in Escherichia coli

The extended N-terminus of Mycobacterium smegmatis RecX potentiates its ability to antagonize RecA functions

The members of the RecX family of proteins have a unique capacity to regulate the catalytic activities of RecA/Rad51 proteins in both prokaryotic and eukaryotic organisms. However, our understanding of the functional roles of RecX in pathogenic and non-pathogenic mycobacteria has been limited by insufficient knowledge of the molecular mechanisms of its activity and regulation. Moreover, the significance of a unique 14 amino acid N-terminal extension in Mycobacterium smegmatis RecX (MsRecX) to its function remains unknown. Here, we advance our understanding of the antagonistic roles of mycobacterial RecX proteins and the functional significance of the extended N-terminus of MsRecX. The full-length MsRecX acts as an antagonist of RecA, negatively regulating RecA promoted functions, including DNA strand exchange, LexA cleavage and ATP hydrolysis, but not binding of ATP. The N-terminally truncated MsRecX variants retain the RecA inhibitory activity, albeit with lower efficiencies compared to the full-length protein. Perhaps most importantly, direct visualization of RecA nucleoprotein filaments, which had been incubated with RecX proteins, showed that they promote disassembly of nucleoprotein filaments primarily within the filaments. In addition, interaction of RecX proteins with the RecA nucleoprotein filaments results in the formation of stiff and irregularly shaped nucleoprotein filaments. Thus, these findings add an additional mechanism by which RecX disassembles RecA nucleoprotein filaments. Overall, this study provides strong evidence for the notion that the N-terminal 14 amino acid region of MsRecX plays an important role in the negative regulation of RecA functions and new insights into the molecular mechanism underlying RecX function.

Characterization of an archaeal recombinase paralog that exhibits novel anti-recombinase activity

The process of homologous recombination is heavily dependent on the RecA family of recombinases for repair of DNA double-strand breaks. These recombinases are responsible for identifying homologies and forming heteroduplex DNA between substrate ssDNA and dsDNA templates, activities that are modified by various accessory factors. In this work we describe the biochemical functions of the SsoRal2 recombinase paralog from the crenarchaeon Sulfolobus solfataricus. We found that the SsoRal2 protein is a DNA-independent ATPase that, unlike the other S. solfataricus paralogs, does not bind either ss- or dsDNA. Instead, SsoRal2 alters the ssDNA binding activity of the SsoRadA recombinase in conjunction with another paralog, SsoRal1. In the presence of SsoRal1, SsoRal2 has a modest effect on strand invasion but effectively abrogates strand exchange activity. Taken together, these results indicate that SsoRal2 assists in nucleoprotein filament modulation and control of strand exchange in S. solfataricus.

Elucidating the functional role of Mycobacterium smegmatis recX in stress response

The RecX protein has attracted considerable interest because the recX mutants exhibit multiple phenotypes associated with RecA functions. To further our understanding of the functional relationship between recA and recX, the effect of different stress treatments on their expression profiles, cell yield and viability were investigated. A significant correlation was found between the expression of Mycobacterium smegmatis recA and recX genes at different stages of growth, and in response to different stress treatments albeit recX exhibiting lower transcript and protein abundance at the mid-log and stationary phases of the bacterial growth cycle. To ascertain their roles in vivo, a targeted deletion of the recX and recArecX was performed in M. smegmatis. The growth kinetics of these mutant strains and their sensitivity patterns to different stress treatments were assessed relative to the wild-type strain. The deletion of recA affected normal cell growth and survival, while recX deletion showed no significant effect. Interestingly, deletion of both recX and recA genes results in a phenotype that is intermediate between the phenotypes of the ΔrecA mutant and the wild-type strain. Collectively, these results reveal a previously unrecognized role for M. smegmatis recX and support the notion that it may regulate a subset of the yet unknown genes involved in normal cell growth and DNA-damage repair.

Diguanylate cyclase null mutant reveals that C-Di-GMP pathway regulates the motility and adherence of the extremophile bacterium Acidithiobacillus caldus

An understanding of biofilm formation is relevant to the design of biological strategies to improve the efficiency of the bioleaching process and to prevent environmental damages caused by acid mine/rock drainage. For this reason, our laboratory is focused on the characterization of the molecular mechanisms involved in biofilm formation in different biomining bacteria. In many bacteria, the intracellular levels of c-di-GMP molecules regulate the transition from the motile planktonic state to sessile community-based behaviors, such as biofilm development, through different kinds of effectors. Thus, we recently started a study of the c-di-GMP pathway in several biomining bacteria including Acidithiobacillus caldus. C-di-GMP molecules are synthesized by diguanylate cyclases (DGCs) and degraded by phosphodiesterases (PDEs). We previously reported the existence of intermediates involved in c-di-GMP pathway from different Acidithiobacillus species. Here, we report our work related to At. caldus ATCC 51756. We identified several putative-ORFs encoding DGC and PDE and effector proteins. By using total RNA extracted from At. caldus cells and RT-PCR, we demonstrated that these genes are expressed. We also demonstrated the presence of c-di-GMP by mass spectrometry and showed that genes for several of the DGC enzymes were functional by heterologous genetic complementation in Salmonella enterica serovar Typhimurium mutants. Moreover, we developed a DGC defective mutant strain (Δc1319) that strongly indicated that the c-di-GMP pathway regulates the swarming motility and adherence to sulfur surfaces by At. caldus. Together, our results revealed that At. caldus possesses a functional c-di-GMP pathway which could be significant for ores colonization during the bioleaching process.

An ArsR/SmtB family member is involved in the regulation by arsenic of the arsenite oxidase operon in Thiomonas arsenitoxydans

The genetic organization of the aioBA operon, encoding the arsenite oxidase of the moderately acidophilic and facultative chemoautotrophic bacterium Thiomonas arsenitoxydans, is different from that of the aioBA operon in the other arsenite oxidizers, in that it encodes AioF, a metalloprotein belonging to the ArsR/SmtB family. AioF is stabilized by arsenite, arsenate, or antimonite but not molybdate. Arsenic is tightly attached to AioF, likely by cysteine residues. When loaded with arsenite or arsenate, AioF is able to bind specifically to the regulatory region of the aio operon at two distinct positions. In Thiomonas arsenitoxydans, the promoters of aioX and aioB are convergent, suggesting that transcriptional interference occurs. These results indicate that the regulation of the aioBA operon is more complex in Thiomonas arsenitoxydans than in the other aioBA containing arsenite oxidizers and that the arsenic binding protein AioF is involved in this regulation. On the basis of these data, a model to explain the tight control of aioBA expression by arsenic in Thiomonas arsenitoxydans is proposed.

Repression of recA induction by RecX is independent of the RecA protein in Deinococcus radiodurans

Besides inhibiting RecA activity at the protein level, Deinococcus radiodurans RecX can suppress RecA induction at the transcriptional level. The regulation of RecX on recA induction is independent of RecA activity, and its N terminus is involved in this process.

Characterization of Biofilm Formation by the Bioleaching Acidophilic Bacterium Acidithiobacillus Ferrooxidans by a Microarray Transcriptome Analysis

Bioleaching is the extraction of metals, such as copper or gold, from ore by microorganisms. Bacterial attachment increases leaching activities due to the formation of a "reaction space" between the metal sulfide surface and the cell. This process depends on abiotic characteristics such as purity and degree of crystallization of the metal sulfide, as well as biotic ones such as the capacity of the bacteria for detecting favourable attachment sites and synthesizing a suitable cell envelope (EPS), for adhesion. Planktonic and sessile cells should differ significantly in their metabolic activities and therefore in their gene expression patterns. To help to understand At. ferrooxidans biofilm formation, microarray transcript profiling was carried out to compare planktonic and sessile cells. The high contents of EPS and ferric iron of the biofilms are interfering with RNA extraction, causing inhibition of DNAse, reverse transcriptase and/or polymerase activities required to get labelled target cDNA. In order to have sufficient high quality RNA suitable for transcriptomic analysis, we have optimized the biofilm formation of At. ferrooxidans on pyrite (FeS2) and the RNA extraction from the sessile cell population. DNA microarrays have been hybridized with labelled cDNAs from sessile and planktonic cells and preliminary data suggest that some genes are differently expressed between these two subpopulations. The understanding of these differences may help us to shift populations of leaching bacteria from the planktonic state towards the sessile state in order to influence bioleaching.

Arsenite oxidation by a chemoautotrophic moderately acidophilic Thiomonas sp.: from the strain isolation to the gene study

An autotrophic bacterium able to gain energy from the oxidation of arsenite was isolated from arsenite-containing acid mine drainage waters. It belongs to the genus Thiomonas as shown by DNA-DNA hybridization experiments, 16S rRNA gene sequence, quinone and fatty acid content analyses. Carboxysomes were observed and the cbbSL genes encoding the ribulose 1,5-bisphosphate carboxylase/oxygenase were detected, confirming that this bacterium is able to fix CO(2). Arsenite oxidation was catalysed by a membrane-bound enzyme, and this activity was detected essentially in cells grown in the presence of arsenite. The genes encoding the two subunits of the arsenite oxidase of the Thiomonas isolate have been sequenced. The small subunit has a characteristic Tat signal sequence and contains the residues binding the [2Fe-2S] Rieske-type cluster. The large subunit has the [3Fe-4S] cluster-binding motif as well as the residues proposed to bind arsenite. In addition, most of the residues interacting with the molybdenum cofactor are conserved. The genes encoding both subunits belong to an operon, likely with a gene encoding a cytochrome c. The expression of this operon is greater in cells grown in the presence than in the absence of arsenite, in agreement with a transcriptional regulation in the presence of this metalloid.

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.

Evidence for a functional quorum-sensing type AI-1 system in the extremophilic bacterium Acidithiobacillus ferrooxidans

Acidithiobacillus ferrooxidans is one of the main acidophilic chemolithotrophic bacteria involved in the bioleaching of metal sulfide ores. The bacterium-mineral interaction requires the development of biofilms, whose formation is regulated in many microorganisms by type AI-1 quorum sensing. Here, we report the existence and characterization of a functional type AI-1 quorum-sensing system in A. ferrooxidans. This microorganism produced mainly acyl-homoserine lactones (AHL) with medium and large acyl chains and different C-3 substitutions, including 3-hydroxy-C8-AHL, 3-hydroxy-C10-AHL, C12-AHL, 3-oxo-C12-AHL, 3-hydroxy-C12-AHL, C14-AHL, 3-oxo-C14-AHL, 3-hydroxy-C14-AHL, and 3-hydroxy-C16-AHL. A quorum-sensing genetic locus that includes two open reading frames, afeI and afeR, which have opposite orientations and code for proteins with high levels of similarity to members of the acyl synthase (I) and transcriptional regulator (R) protein families, respectively, was identified. Overexpression of AfeI in Escherichia coli and the associated synthesis of AHLs confirmed that AfeI is an AHL synthase. As determined by reverse transcription-PCR, the afeI and afeR genes were transcribed in A. ferrooxidans. The transcription levels of the afeI gene were higher in cells grown in sulfur and thiosulfate media than in iron-grown cells. Phosphate starvation induced an increase in the transcription levels of afeI which correlated with an increase in AHL levels. Two afe boxes which could correspond to the AfeR binding sites were identified upstream of the afeI gene. This is the first report of a functional type AI-1 quorum-sensing system in an acidophilic chemolithotrophic microorganism, and our results provide a very interesting opportunity to explore the control and regulation of biofilm formation during the bioleaching process.

Identification of putative sulfurtransferase genes in the extremophilic Acidithiobacillus ferrooxidans ATCC 23270 genome: structural and functional characterization of the proteins

Eight nucleotide sequences containing a single rhodanese domain were found in the Acidithiobacillus ferrooxidans ATCC 23270 genome: p11, p14, p14.3, p15, p16, p16.2, p21, and p28. Amino acids sequence comparisons allowed us to identify the potentially catalytic Cys residues and other highly conserved rhodanese family features in all eight proteins. The genomic contexts of some of the rhodanese-like genes and the determination of their expression at the mRNA level by using macroarrays suggested their implication in sulfur oxidation and metabolism, formation of Fe-S clusters or detoxification mechanisms. Several of the putative rhodanese genes were successfully isolated, cloned and overexpressed in E. coli and their thiosulfate:cyanide sulfurtransferase (TST) and 3-mercaptopyruvate/cyanide sulfurtransferase (MST) activities were determined. Based on their sulfurtransferase activities and on structural comparisons of catalytic sites and electrostatic potentials between homology- modeled A. ferrooxidans rhodaneses and the reported crystal structures of E. coli GlpE (TST) and SseA (MST) proteins, two of the rhodanese-like proteins (P15 and P16.2) could clearly be defined as TSTs, and P14 and P16 could possibly correspond to MSTs. Nevertheless, several of the eight A. ferrooxidans rhodanese-like proteins may have some different functional activities yet to be discovered.

Identification of a gene cluster for the formation of extracellular polysaccharide precursors in the chemolithoautotroph Acidithiobacillus ferrooxidans

A cluster of five genes, proposed to be involved in the formation of extracellular polysaccharide (EPS) precursors via the Leloir pathway, have been identified in the acidophilic autotroph Acidithiobacillus ferrooxidans. The order of the genes is luxA-galE-galK-pgm-galM, encoding a LuxA-like protein, UDP-glucose 4-epimerase, galactokinase, phosphoglucomutase, and galactose mutarotase, respectively. The gal cluster forms a single transcriptional unit and is therefore an operon. Two other putative genes of the Leloir pathway, galU, potentially encoding UDP-glucose pyrophosphorylase, and a gene designated galT-like, which may encode a galactose-1-phosphate uridylyltransferase-like activity, were found unlinked in the genome. Using semiquantitative reverse transcription-PCR, the genes of the gal operon were shown to be expressed more during growth in iron medium than in growth in sulfur medium. The functions of galE, pgm, galU, and the galT-like gene were validated by complementation of Escherichia coli mutants and by in vitro enzyme assays. The data suggest that A. ferrooxidans is capable of synthesizing the EPS precursors UDP-glucose and UDP-galactose. In addition, genes rfbA, -B, -C, and -D were identified in the genome of A. ferrooxidans, suggesting that it can also synthesize the EPS precursor dTDP-rhamnose. Since EPSs constitute the major bulk of biofilms, this study may provide an initial model for the metabolic pathways involved in biofilm formation in A. ferrooxidans and aid in understanding the role of biofilms in mineral leaching and the formation of acid mine drainage.

Expression, purification, and DNA-binding activity of the Herbaspirillum seropedicae RecX protein

The Herbaspirillum seropedicae RecX protein participates in the SOS response: a process in which the RecA protein plays a central role. The RecX protein of the H. seropedicae, fused to a His-tag sequence (RecX His-tagged), was over-expressed in Escherichia coli and purified by metal-affinity chromatography to yield a highly purified and active protein. DNA band-shift assays showed that the RecX His-tagged protein bound to both circular and linear double-stranded DNA and also to circular single-stranded DNA. The apparent affinity of RecX for DNA decreased in the presence of Mg(2+) ions. The ability of RecX to bind DNA may be relevant to its function in the SOS response.

Differential protein expression during growth of Acidithiobacillus ferrooxidans on ferrous iron, sulfur compounds, or metal sulfides

A set of proteins that changed their levels of synthesis during growth of Acidithiobacillus ferrooxidans ATCC 19859 on metal sulfides, thiosulfate, elemental sulfur, and ferrous iron was characterized by using two-dimensional polyacrylamide gel electrophoresis. N-terminal amino acid sequencing and mass spectrometry analysis of these proteins allowed their identification and the localization of the corresponding genes in the available genomic sequence of A. ferrooxidans ATCC 23270. The genomic context around several of these genes suggests their involvement in the energetic metabolism of A. ferrooxidans. Two groups of proteins could be distinguished. The first consisted of proteins highly upregulated by growth on sulfur compounds (and downregulated by growth on ferrous iron): a 44-kDa outer membrane protein, an exported 21-kDa putative thiosulfate sulfur transferase protein, a 33-kDa putative thiosulfate/sulfate binding protein, a 45-kDa putative capsule polysaccharide export protein, and a putative 16-kDa protein of unknown function. The second group of proteins comprised those downregulated by growth on sulfur (and upregulated by growth on ferrous iron): rusticyanin, a cytochrome c(552), a putative phosphate binding protein (PstS), the small and large subunits of ribulose biphosphate carboxylase, and a 30-kDa putative CbbQ protein, among others. The results suggest in general a separation of the iron and sulfur utilization pathways. Rusticyanin, in addition to being highly expressed on ferrous iron, was also newly synthesized, as determined by metabolic labeling, although at lower levels, during growth on sulfur compounds and iron-free metal sulfides. During growth on metal sulfides containing iron, such as pyrite and chalcopyrite, both proteins upregulated on ferrous iron and those upregulated on sulfur compounds were synthesized, indicating that the two energy-generating pathways are induced simultaneously depending on the kind and concentration of oxidizable substrates available.

Regulation of the expression of the Acidithiobacillus ferrooxidans rus operon encoding two cytochromes c, a cytochrome oxidase and rusticyanin

The regulation of the expression of the rus operon, proposed to encode an electron transfer chain from the outer to the inner membrane in the obligate acidophilic chemolithoautroph Acidithiobacillus ferrooxidans, has been studied at the RNA and protein levels. As observed by Northern hybridization, real-time PCR and reverse transcription analyses, this operon was more highly expressed in ferrous iron- than in sulfur-grown cells. Furthermore, it was shown by immunodetection that components of this respiratory chain are synthesized in ferrous iron- rather than in sulfur-growth conditions. Nonetheless, weak transcription and translation products of the rus operon were detected in sulfur-grown cells at the early exponential phase. The results strongly support the notion that rus-operon expression is induced by ferrous iron, in agreement with the involvement of the rus-operon-encoded products in the oxidation of ferrous iron, and that ferrous iron is used in preference to sulfur.

Molecular modeling of RecX reveals its mode of interaction with RecA

The protein RecA is involved in homologous recombination, DNA repair and also catalyzes DNA strand exchange. RecX gene is downstream of recA and the gene product RecX is supposed to be important for RecA regulation. Recombinant RecX is purified to homogeneity, and circular dichroism (CD) and FTIR spectroscopy show the protein to exist mostly in helical conformation. The fluorescence emission maxima of the native and the denatured protein and the steady-state fluorescence quenching studies with acrylamide indicate the presence of tryptophan residues partially exposed to the bulk solvent. Denaturation studies with urea and guanidine hydrochloride by use of spectroscopic methods, fluorescence, and CD also confirm the instability of the protein and unfolding occurs following a two-state model. Mass spectrometry and gel permeation chromatography suggest the monomeric form of the protein. Molecular modeling of RecX represents the molecule as extended and helical bundle in conformity with the spectroscopic results. To understand the mechanism of RecX in the regulation of RecA the structural model of RecA-RecX has been discussed. In this proposed model, entry of RecX into hexameric RecA filament prevents binding of ssDNA and also inhibits ATPase activity.

RecX protein abrogates ATP hydrolysis and strand exchange promoted by RecA: insights into negative regulation of homologous recombination

In many eubacteria, coexpression of recX with recA is essential for attenuation of the deleterious effects of recA overexpression; however, the molecular mechanism has remained enigmatic. Here, we show that Mycobacterium tuberculosis RecX binds directly to M. tuberculosis RecA as well as M. smegmatis and E. coli RecA proteins in vivo and in vitro, but not single-stranded DNA binding protein. The direct association of RecX with RecA failed to regulate the specificity or extent of binding of RecA either to DNA or ATP, ligands that are central to activation of its functions. Significantly, RecX severely impeded ATP hydrolysis and the generation of heteroduplex DNA promoted by homologous, as well as heterologous, RecA proteins. These findings reveal a mode of negative regulation of RecA, and imply that RecX might act as an anti-recombinase to quell inappropriate recombinational repair during normal DNA metabolism.

Construction and characterization of a recA mutant of Thiobacillus ferrooxidans by marker exchange mutagenesis

To construct Thiobacillus ferrooxidans mutants by marker exchange mutagenesis, a genetic transfer system is required. The transfer of broad-host-range plasmids belonging to the incompatibility groups IncQ (pKT240 and pJRD215), IncP (pJB3Km1), and IncW (pUFR034) from Escherichia coli to two private T. ferrooxidans strains (BRGM1 and Tf-49) and to two collection strains (ATCC 33020 and ATCC 19859) by conjugation was analyzed. To knock out the T. ferrooxidans recA gene, a mobilizable suicide plasmid carrying the ATCC 33020 recA gene disrupted by a kanamycin resistance gene was transferred from E. coli to T. ferrooxidans ATCC 33020 by conjugation under the best conditions determined. The two kanamycin-resistant clones, which have retained the kanamycin-resistant phenotype after growth for several generations in nonselective medium, were shown to have the kanamycin resistance gene inserted within the recA gene, indicating that the recA::Omega-Km mutated allele was transferred from the suicide plasmid to the chromosome by homologous recombination. These mutants exhibited a slightly reduced growth rate and an increased sensitivity to UV and gamma irradiation compared to the wild-type strain. However, the T. ferrooxidans recA mutants are less sensitive to these physical DNA-damaging agents than the recA mutants described in other bacterial species, suggesting that RecA plays a minor role in DNA repair in T. ferrooxidans.

Characterization of an operon encoding two c-type cytochromes, an aa(3)-type cytochrome oxidase, and rusticyanin in Thiobacillus ferrooxidans ATCC 33020

Despite the importance of Thiobacillus ferrooxidans in bioremediation and bioleaching, little is known about the genes encoding electron transfer proteins implicated in its energetic metabolism. This paper reports the sequences of the four cox genes encoding the subunits of an aa(3)-type cytochrome c oxidase. These genes are in a locus containing four other genes: cyc2, which encodes a high-molecular-weight cytochrome c; cyc1, which encodes a c(4)-type cytochrome (c(552)); open reading frame 1, which encodes a putative periplasmic protein of unknown function; and rus, which encodes rusticyanin. The results of Northern and reverse transcription-PCR analyses indicated that these eight genes are cotranscribed. Two transcriptional start sites were identified for this operon. Upstream from each of the start sites was a sigma70-type promoter recognized in Escherichia coli. While transcription in sulfur-grown T. ferrooxidans cells was detected from the two promoters, transcription in ferrous-iron-grown T. ferrooxidans cells was detected only from the downstream promoter. The cotranscription of seven genes encoding redox proteins suggests that all these proteins are involved in the same electron transfer chain; a model taking into account the biochemistry and the genetic data is discussed.

Sequence and expression of the rusticyanin structural gene from Thiobacillus ferrooxidans ATCC33020 strain

The periplasmic blue copper protein rusticyanin is thought to play an important role in iron oxidation by Thiobacillus ferrooxidans. We present the sequence of the gene, rus, encoding rusticyanin together with about 1.4 kb of upstream and 0.3 kb of downstream DNA. The rus gene is unique to T. ferrooxidans. Evidence is presented that it is the last gene of an operon and that it can be transcribed from its own promoter. In ATCC33020 strain, rusticyanin is synthesized in ferrous iron but also in sulfur growth conditions suggesting that it could play a role in both energetic metabolisms. The rus gene transcribed from a vector promoter in Escherichia coli leads to the production of a processed aporusticyanin in the periplasmic space, indicating that its signal sequence is correctly recognized by the secretion machinery and the signal peptidase of E. coli.

Characterization and expression of the co-transcribed cyc1 and cyc2 genes encoding the cytochrome c4 (c552) and a high-molecular-mass cytochrome c from Thiobacillus ferrooxidans ATCC 33020

The sequence of the cyc1 gene encoding the Thiobacillus ferrooxidans ATCC 33020 c552 cytochrome, shows that this cytochrome is a 21-kDa periplasmic c4-type cytochrome containing two similar monohaem domains. The kinetics of reduction and the fact that cytochromes c4 are considered to be physiological electron donors of cytochrome oxidases suggest that the last steps of the iron respiratory chain are: rusticyanin-->cytochrome c4-->cytochrome oxidase. In Thiobacillus ferrooxidans, cyc1 is co-transcribed with the cyc2 gene, encoding a high-molecular-mass monohaem cytochrome c. This suggests that the cytochromes encoded by these genes belong to the same electron transfer chain.