3.8 Manganese Oxidation: A Genomic Perspective
In the recent years, several studies were attempted by researchers to understand
the genetics involved in bacterial Mn
2+ oxidation. Marine bacteria are efficient
Mn oxidizers; however, only few studies report the genetic mechanism(s) involved
in Mn oxidation. The well-studied Mn-oxidizing bacteria is Bacillus sp. Strain
SG-1, a marine gram-positive bacterium isolated from shallow marine sediment
that produces Mn-oxidizing spores (van Wassbergen et al. 1993, 1996; Francis et al.
2002; Francis and Tebo 2002). This is the only organism for which the direct
involvement of MCOs in Mn oxidation is established. They proposed MnxG as
one of the first gene products ever shown to be associated with the exosporium
possessing oxidase activity. Francis et al. (2002) demonstrated that MnxG is
localized to the exosporium of wild-type spores and is absent in the nonoxidizing
spores of transposon mutants within the mnx gene cluster. Dick et al. (2006) based
on phylogenetic analysis of 16S rRNA and mnxG genes explained that Mnoxidizing Bacillus sp isolated from Guaymas Basin resembled deep-sea isolates
reported earlier from coastal sediments, with few representing novel strains and
clusters. Recently, Mayhew et al. (2008) proposed that vertical inheritance and gene
loss influenced the distribution of the gene mnxG among the Bacillus sp.
For the first time, van Waasbergen et al. (1993) identified the genes involved in
Mn
2+ oxidation. They demonstrated that mnx region encodes factors that are
required for oxidation of Mn
2+ by SG-1 spores by protoplast transformation and
mutagenesis. Later, van Waasbergen et al. (1996) suggested that among the several
genes (mnxA to mnxG) that were earlier proposed to be involved in Mn
2+ oxidation,
the mnxG gene product may function like a copper oxidase and would be directly
responsible for the oxidation of Mn
2+ by the bacterial spores. The first direct
evidence for the presence of RubisCo genes in a gram-negative Mn-oxidizing
bacterium strain S185-9A1 was given by Caspi et al. (1996). The genes were
more related to those from non-chlorophyte algal chloroplasts than from bacteria.
Dick et al. (2008b) suggested that MnxG catalyzes two sequential one-electron
oxidations from Mn
2+ to Mn
3+ and from Mn
3+ to Mn
4+ , a novel type of reaction for
a multicopper oxidase.
Aurantimonas manganoxydans Strain. SI85-9A1 (Dick et al. 2008a, Anderson
et al. 2009a, b) and Erythrobacter sp Strain. SD-21 (Anderson et al. 2009b) are the
two other marine Mn-oxidizing a-proteobacteria known to oxidize Mn
2+ that have
been recently studied in detail. Anderson et al. (2009b) identified five annotated
MCOs in the genome sequence of the above strains but none of the MCOs were
reported to have any role in Mn
2+ oxidation. In contrast, they could illustrate the
role of heme peroxidase in Mn
2+ oxidation and tentatively suggested MopA for the
putative Ca
2+ binding heme peroxidase.
Leptothrix discophora SS-1 a freshwater bacterial species that deposits Mn
oxides on its extracellular sheath was studied in detail by Corstjens et al. (1997)
and Brouwers et al. (2000a) using sophisticated molecular tools. Based on the
results, they proposed that MCOs like gene mofA (manganese-oxidizing factor) to
be involved in Mn
2+ oxidation and genes mofB and mofC to be a part of the same
60
P.P. Sujith and P.A. Loka Bharathi
In the recent years, several studies were attempted by researchers to understand
the genetics involved in bacterial Mn
2+ oxidation. Marine bacteria are efficient
Mn oxidizers; however, only few studies report the genetic mechanism(s) involved
in Mn oxidation. The well-studied Mn-oxidizing bacteria is Bacillus sp. Strain
SG-1, a marine gram-positive bacterium isolated from shallow marine sediment
that produces Mn-oxidizing spores (van Wassbergen et al. 1993, 1996; Francis et al.
2002; Francis and Tebo 2002). This is the only organism for which the direct
involvement of MCOs in Mn oxidation is established. They proposed MnxG as
one of the first gene products ever shown to be associated with the exosporium
possessing oxidase activity. Francis et al. (2002) demonstrated that MnxG is
localized to the exosporium of wild-type spores and is absent in the nonoxidizing
spores of transposon mutants within the mnx gene cluster. Dick et al. (2006) based
on phylogenetic analysis of 16S rRNA and mnxG genes explained that Mnoxidizing Bacillus sp isolated from Guaymas Basin resembled deep-sea isolates
reported earlier from coastal sediments, with few representing novel strains and
clusters. Recently, Mayhew et al. (2008) proposed that vertical inheritance and gene
loss influenced the distribution of the gene mnxG among the Bacillus sp.
For the first time, van Waasbergen et al. (1993) identified the genes involved in
Mn
2+ oxidation. They demonstrated that mnx region encodes factors that are
required for oxidation of Mn
2+ by SG-1 spores by protoplast transformation and
mutagenesis. Later, van Waasbergen et al. (1996) suggested that among the several
genes (mnxA to mnxG) that were earlier proposed to be involved in Mn
2+ oxidation,
the mnxG gene product may function like a copper oxidase and would be directly
responsible for the oxidation of Mn
2+ by the bacterial spores. The first direct
evidence for the presence of RubisCo genes in a gram-negative Mn-oxidizing
bacterium strain S185-9A1 was given by Caspi et al. (1996). The genes were
more related to those from non-chlorophyte algal chloroplasts than from bacteria.
Dick et al. (2008b) suggested that MnxG catalyzes two sequential one-electron
oxidations from Mn
2+ to Mn
3+ and from Mn
3+ to Mn
4+ , a novel type of reaction for
a multicopper oxidase.
Aurantimonas manganoxydans Strain. SI85-9A1 (Dick et al. 2008a, Anderson
et al. 2009a, b) and Erythrobacter sp Strain. SD-21 (Anderson et al. 2009b) are the
two other marine Mn-oxidizing a-proteobacteria known to oxidize Mn
2+ that have
been recently studied in detail. Anderson et al. (2009b) identified five annotated
MCOs in the genome sequence of the above strains but none of the MCOs were
reported to have any role in Mn
2+ oxidation. In contrast, they could illustrate the
role of heme peroxidase in Mn
2+ oxidation and tentatively suggested MopA for the
putative Ca
2+ binding heme peroxidase.
Leptothrix discophora SS-1 a freshwater bacterial species that deposits Mn
oxides on its extracellular sheath was studied in detail by Corstjens et al. (1997)
and Brouwers et al. (2000a) using sophisticated molecular tools. Based on the
results, they proposed that MCOs like gene mofA (manganese-oxidizing factor) to
be involved in Mn
2+ oxidation and genes mofB and mofC to be a part of the same
60
P.P. Sujith and P.A. Loka Bharathi