Functional environmental proteomics: elucidating the role of a c-type cytochrome abundant during uranium bioremediation

Abstract

Studies with pure cultures of dissimilatory metal-reducing microorganisms have demonstrated that outer-surface c-type cytochromes are important electron transfer agents for the reduction of metals, but previous environmental proteomic studies have typically not recovered cytochrome sequences from subsurface environments in which metal reduction is important. Gel-separation, heme-staining and mass spectrometry of proteins in groundwater from in situ uranium bioremediation experiments identified a putative c-type cytochrome, designated Geobactersubsurface c-type cytochrome A (GscA), encoded within the genome of strain M18, a Geobacter isolate previously recovered from the site. Homologs of GscA were identified in the genomes of other Geobacter isolates in the phylogenetic cluster known as subsurface clade 1, which predominates in a diversity of Fe(III)-reducing subsurface environments. Most of the gscA sequences recovered from groundwater genomic DNA clustered in a tight phylogenetic group closely related to strain M18. GscA was most abundant in groundwater samples in which Geobacter sp. predominated. Expression of gscA in a strain of Geobacter sulfurreducens that lacked the gene for the c-type cytochrome OmcS, thought to facilitate electron transfer from conductive pili to Fe(III) oxide, restored the capacity for Fe(III) oxide reduction. Atomic force microscopy provided evidence that GscA was associated with the pili. These results demonstrate that a c-type cytochrome with an apparent function similar to that of OmcS is abundant when Geobacter sp. are abundant in the subsurface, providing insight into the mechanisms for the growth of subsurface Geobacter sp. on Fe(III) oxide and suggesting an approach for functional analysis of other Geobacter proteins found in the subsurface.

Interspecies electron transfer in syntrophic methanogenic consortia: From cultures to bioreactors

“…Aggregate formation was also much faster in mutants that were deficient in interspecies H 2 transfer. OmcS and other ctype cytochromes are usually localized in form of globules along the pili at roughly equal intervals [75,77], but the exact role that they play in electron transfer is yet unknown. Since deletion of OmcS genes inhibits current production instead of conductivity, it is speculated that OmcS helps transfer electrons from cell to pili but has nothing to do with the pili's properties [78].…”

Characterization of the genome fromGeobacter anodireducens, a strain with enhanced current production in bioelectrochemical systems

“…For example, the multiheme c-type cytochrome GscA (Gbem_3371) found in G. bemidjiensis was able to restore Fe(III) oxide reduction in the OmcS-decient strain of G. sulfurreducens. 96 In addition, while OmcS is required for efficient electron transfer from G. sulfurreducens to Fe(III) oxide, 33 and in the reverse direction from an electron-donating partner during DIET, 79 another extracellular multiheme c-type cytochrome (Gmet_2896) is needed for DIET and Fe(III) reduction by G. metallireducens. 32,40 The outer membrane multiheme c-type cytochromes OmcP (GSU2913) and OmcO (GSU2912) are also not required for reduction of Fe(III) oxide by G. sulfurreducens, 35 but their homologs (Gmet_0557 and Gmet_0558) are required for Fe(III) oxide respiration by G. metallireducens.…”

Uranium Bio-Transformations: Chemical or Biological Processes?

“…Periplasmic cytochrome MacA, a di-heme c-type cytochrome peroxidase also in Geobacter was needed for any U(VI) reduction and up to 30% of the reduced uranium was found accumulated in the periplasm [97]. Recently, proteomics analysis identified one of the Geobacter cytochromes involved in uranium reduction as GscA [100]. In Shewanella, which also a heme-rich organism, outer membrane cytochrome UndA is thought to be involved in metal reductions, including uranium [101].…”