decaheme cytochrome MtrA is only associated with the outer membrane when
MtrB is expressed [135]. Using knock-out mutations and subsequent monitoring of
complex assembly, revealed the existence of a synergetic relationship between
MtrA and MtrB [136]. The assembly of the MtrAB subcomplex stabilizes MtrB,
while subcomplex MtrBC does not assemble in the absence of MtrA. Three other
stable modules similar to MtrAB have been identified in S. oneidensis MR-1. The
MtrDE is proposed to be alternative to MtrAB, the DmsEF is part of porin cytochrome complex specific for DMSO reduction and SO4359-60 forms a secondary
alternative iron reducing pathway to MtrAB [118, 130, 137, 138]. Moreover, gene
clusters encoding for homologous MtrAB modules are phylogenetically distributed
among organisms capable of electron exchange with the extracellular environment
[135, 139]. Both metal-reducing (e.g., Shewanella and Geobacter) and
metal-oxidizing (e.g., Rhodopseudomonas and Sideroxydans) bacteria have
homologous MtrAB modules (Fig. 4) [30, 31, 135, 140–142]. This strengthens the
hypothesis that the MtrAB module is essential for both outwards and inwards EET
[131, 143].
4.3.2 MtrA
Decaheme cytochrome MtrA is a 37 kDa periplasmic cytochrome with 10
bis-histidine low-spin c-type hemes that is associated with the outer membrane via
the integral membrane protein complex MtrCAB [30, 31, 135, 144, 145]. In vivo
cross-linking assays showed that MtrA interacts on the periplasmic side with the
outer membrane ß-barrel protein MtrB [144]. Also, it was shown in vitro that MtrA
forms a stable protein complex with a dissociation constant stronger than 0.1 lM
with its outer membrane partners, MtrB and MtrC [135]. However, under different
experimental conditions MtrA was found to be present in the soluble periplasmic
fraction hinting to weaker affinity for MtrB [108, 145]. In vitro studies showed that
MtrA can interact and receive electrons from the periplasmic cytochrome FccA [97,
108] and also interact with the periplasmic cytochrome STC [97]. It was also
revealed that MtrA can be directly reduced by CymA [96, 108]. Potentiometric
redox titrations showed that MtrA is active over a potential range from −100 to
−400 mV at pH 7.5 [145].
A high-resolution structure of MtrA has not been reported yet but its aminoacid
sequence shows that it is likely to be evolutionarily related to the structurally
characterized pentaheme cytochrome NrfB [146]. Small-angle X-ray scattering
showed that MtrA is shaped like an extended molecular “wire” with overall
dimensions 104 Å Â 20 Å Â 50 Å [133]. Given that the thickness of the
c
Fig. 4 a MtrAB gene cluster in different electroactive microorganisms and their context in the
genome. Cytochromes and b-barrel membrane proteins (MtrB/PioB homologues) are represented
in black and gray, respectively. b Phylogenetic distribution of MtrAB modules in organisms
known to be electroactive [16, 64]. Maximum-likelihood phylogeny with ModelFinder method
(best model: LG + I + G4) of concatenated sequences of MtrA and MtrB. Bootstrap and SH-Like
Test (-alrt) confidence values (from 1000 replicates each) are shown near each node of the major
splits
228
B. M. Fonseca et al.
MtrB is expressed [135]. Using knock-out mutations and subsequent monitoring of
complex assembly, revealed the existence of a synergetic relationship between
MtrA and MtrB [136]. The assembly of the MtrAB subcomplex stabilizes MtrB,
while subcomplex MtrBC does not assemble in the absence of MtrA. Three other
stable modules similar to MtrAB have been identified in S. oneidensis MR-1. The
MtrDE is proposed to be alternative to MtrAB, the DmsEF is part of porin cytochrome complex specific for DMSO reduction and SO4359-60 forms a secondary
alternative iron reducing pathway to MtrAB [118, 130, 137, 138]. Moreover, gene
clusters encoding for homologous MtrAB modules are phylogenetically distributed
among organisms capable of electron exchange with the extracellular environment
[135, 139]. Both metal-reducing (e.g., Shewanella and Geobacter) and
metal-oxidizing (e.g., Rhodopseudomonas and Sideroxydans) bacteria have
homologous MtrAB modules (Fig. 4) [30, 31, 135, 140–142]. This strengthens the
hypothesis that the MtrAB module is essential for both outwards and inwards EET
[131, 143].
4.3.2 MtrA
Decaheme cytochrome MtrA is a 37 kDa periplasmic cytochrome with 10
bis-histidine low-spin c-type hemes that is associated with the outer membrane via
the integral membrane protein complex MtrCAB [30, 31, 135, 144, 145]. In vivo
cross-linking assays showed that MtrA interacts on the periplasmic side with the
outer membrane ß-barrel protein MtrB [144]. Also, it was shown in vitro that MtrA
forms a stable protein complex with a dissociation constant stronger than 0.1 lM
with its outer membrane partners, MtrB and MtrC [135]. However, under different
experimental conditions MtrA was found to be present in the soluble periplasmic
fraction hinting to weaker affinity for MtrB [108, 145]. In vitro studies showed that
MtrA can interact and receive electrons from the periplasmic cytochrome FccA [97,
108] and also interact with the periplasmic cytochrome STC [97]. It was also
revealed that MtrA can be directly reduced by CymA [96, 108]. Potentiometric
redox titrations showed that MtrA is active over a potential range from −100 to
−400 mV at pH 7.5 [145].
A high-resolution structure of MtrA has not been reported yet but its aminoacid
sequence shows that it is likely to be evolutionarily related to the structurally
characterized pentaheme cytochrome NrfB [146]. Small-angle X-ray scattering
showed that MtrA is shaped like an extended molecular “wire” with overall
dimensions 104 Å Â 20 Å Â 50 Å [133]. Given that the thickness of the
c
Fig. 4 a MtrAB gene cluster in different electroactive microorganisms and their context in the
genome. Cytochromes and b-barrel membrane proteins (MtrB/PioB homologues) are represented
in black and gray, respectively. b Phylogenetic distribution of MtrAB modules in organisms
known to be electroactive [16, 64]. Maximum-likelihood phylogeny with ModelFinder method
(best model: LG + I + G4) of concatenated sequences of MtrA and MtrB. Bootstrap and SH-Like
Test (-alrt) confidence values (from 1000 replicates each) are shown near each node of the major
splits
228
B. M. Fonseca et al.