His/Met heme ligation in the PioA outer membrane cytochrome enabling light-driven extracellular electron transfer by Rhodopseudomonas palustris TIE-1.

A growing number of bacterial species are known to move electrons across their cell envelopes. Naturally this occurs in support of energy conservation and carbon-fixation. For biotechnology it allows electron exchange between bacteria and electrodes in microbial fuel cells and during microbial electrosynthesis. In this context Rhodopseudomonas palustris TIE-1 is of much interest. These bacteria respond to light by taking electrons from their external environment, including electrodes, to drive CO2-fixation. The PioA cytochrome, that spans the bacterial outer membrane, is essential for this electron transfer and yet little is known about its structure and electron transfer properties. Here we reveal the ten c-type hemes of PioA are redox active across the window +250 to -400 mV versus Standard Hydrogen Electrode and that the hemes with most positive reduction potentials have His/Met and His/H2O ligation. These chemical and redox properties distinguish PioA from the more widely studied family of MtrA outer membrane decaheme cytochromes with ten His/His ligated hemes. We predict a structure for PioA in which the hemes form a chain spanning the longest dimension of the protein, from Heme 1 to Heme 10. Hemes 2, 3 and 7 are identified as those most likely to have His/Met and/or His/H2O ligation. Sequence analysis suggests His/Met ligation of Heme 2 and/or 7 is a defining feature of decaheme PioA homologs from over 30 different bacterial genera. His/Met ligation of Heme 3 appears to be less common and primarily associated with PioA homologs from purple non-sulphur bacteria belonging to the alphaproteobacteria class.

Mechanistic insights into the key marine dimethylsulfoniopropionate synthesis enzyme DsyB/DSYB.

Marine algae and bacteria produce approximately eight billion tonnes of the organosulfur molecule dimethylsulfoniopropionate (DMSP) in Earth's surface oceans annually. DMSP is an antistress compound and, once released into the environment, a major nutrient, signaling molecule, and source of climate-active gases. The methionine transamination pathway for DMSP synthesis is used by most known DMSP-producing algae and bacteria. The S-directed S-adenosylmethionine (SAM)-dependent 4-methylthio-2-hydroxybutyrate (MTHB) S-methyltransferase, encoded by the dsyB/DSYB gene, is the key enzyme of this pathway, generating S-adenosylhomocysteine (SAH) and 4-dimethylsulfonio-2-hydroxybutyrate (DMSHB). DsyB/DSYB, present in most haptophyte and dinoflagellate algae with the highest known intracellular DMSP concentrations, is shown to be far more abundant and transcribed in marine environments than any other known S-methyltransferase gene in DMSP synthesis pathways. Furthermore, we demonstrate in vitro activity of the bacterial DsyB enzyme from Nisaea denitrificans and provide its crystal structure in complex with SAM and SAH-MTHB, which together provide the first important mechanistic insights into a DMSP synthesis enzyme. Structural and mutational analyses imply that DsyB adopts a proximity and desolvation mechanism for the methyl transfer reaction. Sequence analysis suggests that this mechanism may be common to all bacterial DsyB enzymes and also, importantly, eukaryotic DSYB enzymes from e.g., algae that are the major DMSP producers in Earth's surface oceans.

Bacteria are important dimethylsulfoniopropionate producers in coastal sediments.

Dimethylsulfoniopropionate (DMSP) and its catabolite dimethyl sulfide (DMS) are key marine nutrients1,2 that have roles in global sulfur cycling2, atmospheric chemistry3, signalling4,5 and, potentially, climate regulation6,7. The production of DMSP was previously thought to be an oxic and photic process that is mainly confined to the surface oceans. However, here we show that DMSP concentrations and/or rates of DMSP and DMS synthesis are higher in surface sediment from, for example, saltmarsh ponds, estuaries and the deep ocean than in the overlying seawater. A quarter of bacterial strains isolated from saltmarsh sediment produced DMSP (up to 73 mM), and we identified several previously unknown producers of DMSP. Most DMSP-producing isolates contained dsyB8, but some alphaproteobacteria, gammaproteobacteria and actinobacteria used a methionine methylation pathway independent of DsyB that was previously only associated with higher plants. These bacteria contained a methionine methyltransferase gene (mmtN)-a marker for bacterial synthesis of DMSP through this pathway. DMSP-producing bacteria and their dsyB and/or mmtN transcripts were present in all of the tested seawater samples and Tara Oceans bacterioplankton datasets, but were much more abundant in marine surface sediment. Approximately 1 × 108 bacteria g-1 of surface marine sediment are predicted to produce DMSP, and their contribution to this process should be included in future models of global DMSP production. We propose that coastal and marine sediments, which cover a large part of the Earth's surface, are environments with high levels of DMSP and DMS productivity, and that bacteria are important producers of DMSP and DMS within these environments.

Membrane-spanning electron transfer proteins from electrogenic bacteria: Production and investigation.

Certain bacterial species have a natural ability to exchange electrons with extracellular redox partners. This behavior allows coupling of catalytic transformations inside bacteria to complementary redox transformations of catalysts and electrodes outside the cell. Electricity generation can be coupled to waste-water remediation. Industrially relevant oxidation reactions can proceed exclusively when electrons are released to anodes. Reduced products such as fuels can be generated when electrons are provided from (photo)cathodes. Rational development of these opportunities and inspiration for novel technologies is underpinned by resolution at the molecular level of pathways supporting electron exchange across bacterial cell envelopes. This chapter describes methods for purification, engineering, and in vitro characterization of proteins providing the primary route for electron transport across the outer membrane lipid bilayer of Shewanella oneidensis MR-1, a well-described electrogenic bacterium and chassis organism for related biotechnologies.

DddW, a third DMSP lyase in a model Roseobacter marine bacterium, Ruegeria pomeroyi DSS-3.

Ruegeria pomeroyi DSS-3 is a model Roseobacter marine bacterium, particularly regarding its catabolism of dimethylsulfoniopropionate (DMSP), an abundant anti-stress molecule made by marine phytoplankton. We found a novel gene, dddW, which encodes a DMSP lyase that cleaves DMSP into acrylate plus the environmentally important volatile dimethyl sulfide (DMS). Mutations in dddW reduced, but did not abolish DMS production. Transcription of dddW was greatly enhanced by pre-growth of cells with DMSP, via a LysR-type regulator. Close DddW homologs occur in only one other Roseobacter species, and there are no close homologs and only a few related sequences in metagenomes of marine bacteria. In addition to DddW, R. pomeroyi DSS-3 had been shown to have two other, different, DMSP lyases, DddP and DddQ, plus an enzyme that demethylates DMSP, emphasizing the importance of this substrate for this model bacterium.