Identification and characterization of proteins supporting dehalorespiration in Desulfitobacterium hafniense strain TCE1

Respiration is a fundamental catalytic process in the aerobic and anaerobic energy metabolism of many prokaryotic and most eukaryotic organisms. The major difference between these organisms is that various organic and inorganic substrates can be used to donate or accept electrons in the bacterial and archaeal kingdoms, in various ranges of redox potentials in order to drive aerobic (with oxygen as electron acceptor) and anaerobic respiratory pathways. During the last few decades, several bacterial strains have been reported to use chlorinated compounds as terminal electron acceptor under anaerobic conditions, in a process called dehalorespiration. Among more than thirty dehalorespiring bacterial strains isolated to date, the Dehalococcoides group and the Gram-positive genus Desulfitobacterium comprise the largest number of reductively dehalogenating pure cultures. While most Dehalococcoides isolates appeared as highly specialized bacteria that depend strictly on dehalorespiration for growth, members of the genus Desulfitobacterium are very versatile microorganisms with regards to the electron donors and acceptors utilized. Globally, this remarkable prokaryotic respiratory flexibility is guaranteed by an elaborate reservoir of complex redox enzymes. Regarding the specific electron transport chain involved in dehalorespiration, rather little knowledge has been obtained so far with the exception of the key enzyme, the reductive dehalogenase, well described in several dehalorespiring bacteria, and of little information on b-type cytochromes and menaquinones. The overall objective of this thesis was to identify new possible components of the electron transport chain by comparative proteomic analysis, to investigate the presence/absence of a regulatory pathway leading to the biosynthesis of the reductive dehalogenase, and finally to characterize newly identified proteins that might support the dehalorespiration process. A comparative 2D proteomic analysis was performed on Desulfitobacterium hafniense strain TCE1, a bacterium capable of using tetrachloroethene (PCE) as electron acceptor. Soluble and membrane-associated proteins were analyzed from cells cultivated on different couples of electron donors and acceptors, lactate – fumarate (LF), lactate – PCE (LP), hydrogen – fumarate (HF), and hydrogen – PCE (HP). Cells growing on LP or HP revealed 72 and 93 protein spots in membrane fraction, and 49 and 12 spots in soluble fraction that correspond to increasingly expressed proteins compared to their fumarate-grown counterparts. More than 80 proteins were identified by mass spectrometry analysis, among which the key enzyme in the dehalorespiration process, the PCE reductive dehalogenase (PceA), as well as interesting candidates which could support dehalorespiration. The possible role of these proteins was analyzed in further details. Involved in energy and carbon metabolisms, electron transfer flavoproteins (ETF) and proteins related to the Wood-Ljungdahl pathway of CO2 fixation were identified in HP growth conditions. Proteins associated to stress response such as the phage shock protein PspA, and to regulatory pathways, the transcriptional repressor CodY, were also observed and discussed. Although only slightly induced by PCE, an additional protein displaying homology to sulfur-transferases was found in a large abundance in the fraction of membrane-associated proteins. This protein was named PhsE, as the corresponding gene belongs to a gene operon with homology to the molybdoenzyme thiosulfate/polysulfide reductase operons (phs/psr) found in Salmonella typhimurium or Wolinella succinogenes. PhsE protein architecture resembles to the class of tandem-domain rhodaneses, with the particularity to contain two active-site cysteines. It is also characterized by the presence of a typical lipoprotein signal peptide (LSP), which anchors PhsE on the outside of the cytoplasmic membrane. In the genome sequence of D. hafniense strain Y51, phsE (DSY3892) is part of an apparent operon encoding one out of about sixty different members of the complex iron-sulfur molybdoenzyme (CISM) family. PhsA, the catalytic molybdoenzyme, displays significant sequence homology with the thiosulfate/polysulfide reductase (PhsA/PsrA) subfamily. So far no other CISM operon has been described with a sulfur-transferase encoding gene. Despite the numerous functions proposed for rhodanese in cyanide detoxification, Fe/S cluster formation, oxidative stress protection, sulfur mobilization and involvement in general sulfur metabolism, the question of the physiological role of PhsE remains open. The expression of the Phs complex appeared to be influenced by the addition of sulfide in the culture medium, generally used as reducing agent in anaerobic cultures. Sulfide is known to affect the intracellular redox status. Therefore, it is hypothesized that PhsE might be involved in the control of the redox balance of the cell, notably by scavenging potentially toxic reduced sulfur species. To support data obtained at the protein level during proteomic analysis, genes and the corresponding messenger RNA from D. hafniense strain TCE1 were studied. While in silico DNA sequence analysis did not reveal any obvious features regulating the transcription of pceA, puzzling results were obtained in proteomic analysis showing an almost complete absence of PceA in cells grown on fumarate. To characterize this phenomenon, starting from a routinely PCE-grown inoculum, strain TCE1 was repeatedly transferred on medium containing fumarate as electron acceptor. The pool of pceA gene in the total culture DNA decreased significantly already after fifteen successive sub-cultures on fumarate, suggesting a shift in strain TCE1 population upon relief of the PCE selection pressure. Consequently the level of pceA mRNA and PceA protein also followed the same trend. Secondly, a PCE pulse experiment was performed in an anaerobic continuous culture of strain TCE1 on fumarate. It revealed that PCE itself has no power of induction on genetic level, except for the positive effect observed on pspA transcripts which encodes a protein involved in cell membrane integrity. Finally the expression study of the phsFABCDE gene cluster revealed that all phs genes are co-transcribed, and most likely build an operon. Thus phsE seemed to be strictly coregulated with the phs operon, and not under the influence of another promoter. In conclusion, proteomic analysis enabled to investigate the general physiological status of Desulfitobacterium hafniense strain TCE1 during dehalorespiration. A tentative model of the dehalorespiration pathway is proposed in summarizing recent data obtained on the topology of the PCE reductive dehalogenase and on sequence similarity with proteins involved in other anaerobic respiratory pathways. Regarding the possible support of PhsE towards the dehalorespiration, the complicated chemistry of sulfur does not allow to clearly attribute a function to the Phs complex, but there are indications that it might build a 'sulfur' oxidoreductase which could catalyze the conversion of sulfide to polysulfide with concomitant electron transfer to the quinone pool. The sulfide oxidation activity might be useful to D. hafniense to scavenge toxic sulfur species, avoiding that they disturb the redox balance of the cell, and redox-sensitive enzymatic complexes.


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