Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 293

periplasmically located, decaheme DmsE cytochrome to the DmsABF complex in
the outer membrane (Figure 7 )[ 166 - 168 ]. The DmsE cytochrome is required for
full activity of the complex, but results indicate that related, MtrA-like cytochromes
such as MtrA, MtrD, SO_4360 or the CctA tetraheme cytochrome may be able to at
least partially substitute for DmsE function [ 169 ].
Due to the outer membrane location of this and other enzyme systems (mediat-
ing, e.g., Fe(III) and Fe(III) citrate reduction) in S. oneidensis , the term 'extracel-
lular respiration' has been coined to describe the function of these respiratory
complexes [ 166 , 170 ]. It has been hypothesized that the location of the DMSOR
in the outer membrane facilitates the use of particulate or substrate-bound DMSO.
Regulation of DMSO respiration in S. oneidensis appears to involve cyclic AMP
(cAMP), cAMP receptor protein (Crp), and an ArcAB-related two-component
regulatory system [ 171 - 173 ].
2.1.3.4 The Halobacterium NRC-1 Dimethylsulfoxide Respiratory System
An interesting observation is that Dms-type respiratory systems appear to also occur
in Archaea, where a dmsR-dmsEABCD operon has been identified in Halobacterium
NRC-1. In this operon the dmsABCD genes encode the subunits of the enzyme and
the required molecular chaperone [ 174 ]. Based on sequence alignments these authors
proposed that the Mo amino acid ligand of the DmsA protein should be an aspartate.
The function of the 82 aa DmsE protein is unknown, it is annotated as a
hypothetical protein and related sequences appear to be confined to the
Halobacteriaceae. Nevertheless, the dmsE gene is transcribed as part of the
DMSO reductase operon in these Archaea, as shown by both transcript mapping
and transcriptome analyses [ 174 ]. The dmsR gene encodes a regulator of the Bat
family of transcriptional activators that is required for expression of the DMSO
reductase operon. The dmsEABCD operon was shown to be induced during anaer-
obic growth and was required for growth on DMSO, but not on TMAO.
According to our analyses, enzymes related to the Halobacterium NRC-1
DMSO reductase are found exclusively in the euryarchaeotic lineage of the
Halobacteriaceae, but close relatives are also present in the firmicutes and high
GC Gram-positive bacteria (32 % aa identity; 50 % aa similarity), which might
indicate an acquisition of the dms operon by the halophilic Archaea from bacterial
lineages. For comparison - between the NRC_1 DmsA and the E. coli DmsA amino
acid sequence similarities are 23 % identity/42 % similarity.
2.2 Dimethylsulfide Dehydrogenases
2.2.1 The Rhodovulum sulfidophilum Dimethylsulfide Dehydrogenase
Like the DMSO reductases, the DMS dehydrogenases belong to the DMSO reductase
enzyme family, however, at present there is only one enzyme of this type that has