2011 - Cooperative Institute for Research in Environmental Sciences ...

Shelley Copley
The Origin of the PCP-Degradation Genes
in Sphingbobium chlorophenolicum L-1
FUNDING: NATIONAL INSTITUTES OF HEALTH
The pesticide pentachlorophenol
(PCP)
is listed as a Priority
Pollutant by the U.S. Environmental
Protection
Agency due to its toxicity
and persistence in the environment.
Sphingobium
chlorophenolicum, a bacterium
isolated from a PCPcontaminated
site, has
patched together a poorly
functioning pathway (left
figure) that allows mineralization
of PCP, although
degradation is slow and
the bacterium cannot
grow at high concentrations
of PCP.
We recently sequenced the genome of S. chlorophenolicum
L-1. Comparison of this sequence with that of the
closely related Sphingobium japonicum, which degrades
lindane, provides insights into the origins of the PCP
degradation genes. The right figure shows a map of
chromosome 2 of S. chlorophenolicum. The locations of the
PCP degradation genes are shown in blue on the second
circle. Orange bars in the third circle show the positions
of genes in S. chlorophenolicum that are also found in S.
japonicum. The inner circle shows guanine-cytosine (GC)
content. Gene segments that are acquired by horizontal
gene transfer often have significantly different GC content.
The genes pcpB and pcpD on one side of the chromosome
and pcpC on the other side have no homologs in
S. japonicum and are located in regions with low GC
content. In contrast, pcpA and pcpE have closely related
homologs in S. japonicum.
Our analysis suggests that the first three enzymes
in the pathway were acquired by S. chlorophenolicum by
horizontal gene transfer in two separate events after
it diverged from S. japonicum. In contrast, the last two
enzymes in the pathway were present in the most recent
common ancestor of S. chlorophenolicum and S. japonicum.
None of the genes encoding the PCP degradation enzymes
arose by duplication and divergence of ancestral genes
within S. chlorophenolicum. The genes occur in two disparate
parts of the genome and have not yet been integrated
into a compact and consistently regulated operon.
pcpE
pcpA
Left, a map of chromosome 2 of S. chlorophenolicum. Right, the PCP degradation pathway.
pcpC
pcpB and pcpD
CHR2
CIRES Annual Report 2011 31