Arsenic resistance genes in nocardioform bacteria
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Date
2008-07-10T08:56:56Z
Authors
Leach, Jacobus Gerhardus Johannes
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Abstract
Arsenic contamination is becoming a global dilemma making it imperative to study natural
biomechanisms involved in arsenic detoxification in humans and other living organisms to gain
insight in ways how arsenic poisoning can be treated. Arsenic resistance genes are being
detected in ever increasing microorganisms with diverse gene organizations. Here, in this
report the presence of ars genes in Rhodococcus rhodochrous, Arthrobacter oxydans, and
Gordonia rubropertincta was investigated. Genomic libraries of these AsR actinomycete species
were constructed and then screened for arsenic resistance and unique carbon source utilization
genes. More than one hundred positive arsenate tolerant clones were obtained from a partially
Pst I digested library of Gordonia rubropertincta of which ten random clones (pKL1-10) clearly
showed the arsenic resistance phenotype in an AsS Gram-positive recipient strain. Nine of these
ten clones demonstrated a conserved 3.5 kbp DNA fragment with only clone 1 containing a 5100
bp nucleotide segment. Restriction mapping and subcloning was performed on the designated
pKL335 and pKL1 recombinant plasmid constructs after which the minimum DNA required for
conferring arsenic resistance were sent for sequencing. Only the pKL316 and pKL120 subclones
were successfully sequenced as putative secondary structures prevented the sequencing of the
rest of pKL335 and pKL1. Four and three putative cistrons were predicted from the edited
contiguous sequences of pKL335(ORF1-4) and pKL120(ORF5-7), respectively. Blastn,x,p and
CD alignment searches revealed that ORF1 was a putative Acr3-type ArsB homolog, ORF2,3,4
and 7 showed significant homology with Trx-type ArsCs, and ORF5 and 6 are flavin-binding
monooxygenase and thioredoxin reductase analogs, respectively. From the alignment data it can
be deduced that pKL335 carries an arsRBCCC operon structure, such as C. efficiens YS-314,
and that pKL1 probably comprises a unique arsRBCCCTO gene organization. This is only the
third report of a thioredoxin reductase and a flavoprotein detected within an arsenic resistant
gene cluster. Multiple ArsCs, an ArsT/TrxB and an ArsO/FMO-like homolog seems to be a
unique feature amongst high GC-rich Gram(+) bacteria.