Recently, studies have been reported in which fluorescently labeled redox proteins have been studied with a combination of spectroscopy and electrochemistry. In order to understand the effect of the dye on the protein-electrode interaction, voltammetry and surface analysis have been performed on protein films of dye-labeled and unlabeled forms of a cysteine-surface variant (L93C) and the wild type (wt) of the copper containing nitrite reductase (NiR) from Alcaligenes faecalis S6. The protein has been adsorbed onto gold electrodes modified with self-assembled monolayers (SAMs) made up of 6-mercaptohexanol (6-OH) and mixtures of various octanethiols. Electrochemical and surface-analytical techniques were utilized to explore the influence of the SAM composition on wt and L93C NiR enzyme activity and the orientation of the enzyme molecules with respect to the electrode/SAM. The unlabeled L93C NiR enzyme is only electroactive on mixed SAMs composed of positive 8-aminooctanethiol (8-NH(2)) and 8-mercaptooctanol (8-OH). No enzymatic activity is observed on SAMs consisting of pure 6-OH, 8-OH, or pure 8-NH(2). Modification of L93C NiR with the ATTO 565 dye resulted in enzymatic activity on SAMs of 6-OH, but not on SAMs of 8-OH. Quartz crystal microbalance with dissipation measurements show that well-ordered and rigid protein films (single orientation of the protein) are formed when NiR is electroactive. By contrast, electrode-NiR combinations for which no electrochemical activity is observed still have NiR adsorbed on the surfaces, but a less-structured and water-rich film is formed. For the unlabeled L93C NiR, bilayer formation is observed, suggesting that the Cys93 residue is orientated away from the surface and able to form disulfide bridges to a second layer of L93C NiR. The results indicate that interfacial electron transfer is only possible if the negatively charged surface patch surrounding the electron-entry site of NiR is directed toward the electrode. This can be achieved either by introducing positive charges in the SAM or, when the SAM does not carry a charge, by labeling the enzyme with an ATTO 565 dye, which has some hydrophobic character, close to the electron entry site of the NiR.

MamA is a highly conserved protein found in magnetotactic bacteria (MTB), a diverse group of prokaryotes capable of navigating according to magnetic fields – an ability known as magnetotaxis. Questions surround the acquisition of this magnetic navigation ability; namely, whether it arose through horizontal or vertical gene transfer. Though its exact function is unknown, MamA surrounds the magnetosome, the magnetic organelle embedding a biomineralised nanoparticle and responsible for magnetotaxis. Several structures for MamA from a variety of species have been determined and show a high degree of structural similarity. By determining the structure of MamA from Desulfovibrio magneticus RS-1 using X-ray crystallography, we have opened up the structure-sequence landscape. As such, this allows us to perform structural- and phylogenetic-based analyses using a variety of previously determined MamA from a diverse range of MTB species across various phylogenetic groups. We found that MamA has remained remarkably constant throughout evolution with minimal change between different taxa despite sequence variations. These findings, coupled with the generation of phylogenetic trees using both amino acid sequences and 16S rRNA, indicate that magnetotaxis likely did not spread via horizontal gene transfer and instead has a significantly earlier, primordial origin.

Supporting Information S1 Fig. Oligomeric state of purified MamAΔ41 according to size exclusion (Superdex 200) chromatograms from different species. Elution profiles of MamAΔ41 triple mutant from Desulfovibrio magneticus (RS-1) and wild type MamAΔ41 from Desulfovibrio magneticus (RS-1), M. magneticum (AMB-1), M.gryphiswaldense (MSR-1) and Candidatus Magnetobacterium bavaricum (Mbav) colored in light blue, green, red, orange and blue, respectively. Wild type MamAΔ41 from RS-1 eluted in a volume corresponds to octamer (~192 kDa) whereas the triple mutated MamAΔ41 eluted in three separate peaks that correspond to a 13-monomer oligomer (~312 kDa), octamer (~192 kDa) and a monomer (~24 kDa). Both MamAΔ41 from AMB-1 and Mbav eluted at a volume corresponding to the monomer (20-22 kDa). MamAΔ41 from MSR-1 eluted at a volume typical of the trimer (~60 kDa). Dashed green line represents the elution profile of protein markers: Ferrritin (~440 kDa), Ovalbumin (~43 kDa), Carbonic Anhydrase (~29 kDa), Ribonuclease (~14 kDa). (TIF)

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