Transformations of Dimethylsulfide - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 285

redox enzyme maturation protein, and is known to play a role in aiding cofactor
insertion and export of DmsA to the periplasm [ 103 - 105 ]. The gene encoding this
protein may be part of the dmsABC operon, as is the case in Haemophilus influenzae
[ 106 ] , however, it can also be encoded elsewhere on the chromosome (Figure 5 ). This
is the case in E. coli where the DmsD protein required for the maturation of the
DmsABC protein complex is part of an operon encoding a putative selenate reductase
( ynfE) that is a paralogue of the DmsABC DMSO reductase [ 100 , 107 - 109 ].
Additional genes encoding redox proteins such as ferredoxins may be present in
the dms operons. This is the case for Haemophilus influenzae , where a gene encoding
a NapF-like ferredoxin is present downstream of the dmsABCD genes. NapF proteins
are normally associated with another type of DMSOR family molybdenum enzymes,
the periplasmic nitrate reductases, and are thought to play a role in the insertion of the
iron-sulfur cluster present in the NapA catalytic subunit [ 110 ].
The Dor-type DMSO reductases were first characterized from photohetero-
trophic Rhodobacter species [ 80 , 111 - 113 ], and the operons encoding these
enzymes usually consist of three genes, dorCDA , where the first gene encodes a
membrane-bound penta-heme cytochrome c , DorC, which acts as the electron
donor to the catalytic subunit, DorA (Figures 4 and 5 ). The second gene encodes
a DMSO reductase-specific chaperone protein that has been named either dorB
( R. sphaeroides )or dorD ( R. capsulatus ), with the latter being the more common
designation. The third gene, dorA , encodes the Mo cofactor containing catalytic
subunit. DorA proteins differ from DmsA proteins in that they do not contain iron-
sulfur clusters [ 114 , 115 ].
The Dor-type DMSO reductases are closely related to the trimethylamine
N-oxide (TMAO) reductases found in many bacteria including E. coli , and the
operon structures of the TMAO reductases are very similar to those of the DMSO
reductases although the order of the genes is usually torCAD [ 116 ].
A number of accessory genes located close to the dorCDA operons have been
identified in the two Rhodobacter species in which these enzymes have been
studied most. Upstream of the dorCDA operon two genes, dorRS , encoding a
two-component regulatory system have been identified in both species. This
two-component regulatory system is involved in regulating expression of the
DMSO reductase structural genes [ 78 , 114 , 115 ].
In Rhodobacter capsulatus 37B4 an additional gene, dorB , encoding a
NapD-like chaperone protein has been identified directly downstream of the
dorCDA operon, and a similar gene appears to be present

in R. capsulatus

SB1003 [ 114 , 115 , 117 ].
Downstream of the dor operons in both Rhodobacter sp., two genes encoding
molybdenum cofactor synthesis proteins, moeA and moaA , are found [ 115 ,
118 ]. The genome sequence of R. capsulatus SB1003 also shows that not only
two but five genes encoding all major proteins involved in the production of the
Mo-pyranopterin cofactor (acc. No YP_003578980-75) are found downstream of
the dor operon [ 117 , 119 ] (Figure 5 ). This is, however, not the case for
R. sphaeroides 2.4.1 where only a moeA and a moaA gene are present
[ 120 , 121 ]. It is possible that more Mo-pyranopterin biosynthesis protein-encoding