Environmental Engineering Reference
In-Depth Information
biphenyl, its growth on 2-CB yielded no dechlorination, only about half as
much protein compared to its growth on 4-CB and biphenyl, and stoichio-
metric amounts of 2-CBA released in the medium. These results confirm the
ability of the parent strain VP44 to utilize pentadiene derived from the
nonchlorinated ring of both the para- and ortho-chlorobiphenyl molecule
for growth. Thus, the introduction of the dehalogenation genes resulted in
recombinant organisms that are capable of complete mineralization of ortho-
and para-chlorobiphenyls.
Growth on biphenyls via the BP pathway for biphenyl degradation
yields a benzoate and pentadiene as key intermediates, either or both of
which may be chlorinated, depending on the PCB congener (Hofer et al.,
1993). Introduction of the
ohb
(pE43) and
fcb
(pPC3) operons controlling
ortho- and para-dechlorination of the aromatic ring, respectively, resulted in
a recombinant pathway for degradation of chlorobiphenyls via dechlorina-
tion of chlorobenzoates. Both recombinant variants VP44(pE43) and
VP44(pPC3) compared favorably to other chlorobiphenyl-degrading recom-
binant strains previously constructed via intergenic mating. Both of our
strains were capable of growth on chlorobenzoates and chlorobiphenyls at
high concentrations of up to 10 m
M
. By comparison, strain M3GY (McCullar
et al., 1994) was grown on 0.89 m
M
3-4′-CB with a doubling time of approx-
imately 20 days. Strain JHR22 (Havel and Reineke, 1991) was reported to
grow on several chlorobiphenyls, including 2-CB and 4-CB, and exhibited
doubling times of approximately 16 and 10 h. Another strain, UCR2 (Hickey
et al., 1992), was reported to mineralize both 2-CB and 2,5-CB, with doubling
times of 20 and 48 h. Introduction of specific dechlorination genes can permit
growth on otherwise recalcitrant substrates and may be more easily accom-
plished than methods previously used to generate recombinant PCB degrad-
ers. Each recombinant strain was also tested for growth on other PCB con-
geners. Strain VP44(pE43) grew on plates with 2,2′- and
2,4′-dichlorobiphenyls, and VP44(pPC3) grew on plates with 4,4′- and
2,4′-chlorobiphenyls, the congeners expected to yield either 2-CBA or 4-CBA
as intermediates (Pellizari et al., 1996). To our knowledge, none of the pre-
viously constructed PCB-growing strains grew on congeners with both sub-
stituted rings.
No growth was observed when parent VP44 cultures were amended
with chlorobiphenyls containing halogen atoms in both rings of biphenyl
moiety, such as 2,2′- and 2,4′-CB, suggesting the strain does not possess a
pathway for efficient oxidation of chlorinated pentadiene. Apparently, this
is a common characteristic for natural biphenyl-degrading bacteria that can
grow on monochlorobiphenyls via oxidation of a nonchlorinated ring pro-
ducing chlorobenzoate as a final product but do not grow on PCB congeners
with both chlorinated aromatic rings (Ahmed and Focht, 1973; Brenner et
al., 1994). One of the exceptions is
Pseudomonas
strain MB86 (Barton and
Crawford, 1988), which was isolated on 4-CBA and grew poorly on 4-CB,
probably due to toxicity of 4-chloroacetophenone, which is formed as an
intermediate from 4-CB. Chlorinated pentadienes presumably generated
