cleavage to produce 2-OH-HOPDA [2- hydroxy-6-(2-hydroxyphenyl)-6-oxo-hexa2,4-dienoic acid] (Strubel et al. 1991). The 2-OH-HOPDA is hydrolysed by a
hydrolase (2-hydroxy-6-oxo-6-phenylexa-2,4-dienate) to form salicylate and 2hydroxypenta-2,4-dienoate (Bunz and Cook 1993). This hydrolysis is same as
seen during bacterial metabolism of biphenyl (hydrolysis of 2-hydroxy-6-oxo-6phenyl-hexa-2,4-dienoate). However, the ring-cleavage product 6-(2hydroxyphenyl)ester of 2-hydroxymuconic acid formed from 2,2
0 ,3trihydroxydiphenyl ether undergoes hydrolysis to form 2-hydroxymuconate and
catechol (Wittich et al. 1992).
Thus, hydrolysis of DF and dioxin by enzymes form salicylate and 2hydroxypenta-2,4-dienoate or catechol and 2-hydroxymuconate. During degradation, either meta-cleavage pathway or ortho-cleavage pathway of catechol is
induced. Meta-cleavage pathway of catechol induced during degradation of DF
and dibenzo-p-dioxin DD by RW1 forms two intermediates 2-hydroxymuconate
and 2- hydroxypenta-2, 4-dienoate (Whitman et al. 1992; Lian and Chapman 1993).
On the other hand, ortho-cleavage of catechol is induced during DD degradation.
Thus, the degradation of catechol can be accomplished by either meta-cleavage or
ortho-cleavage; however, there is paucity of information about relative importance
of these routes. During DF degradation, catechol meta-cleavage and gentisate
pathways are induced. However, degradation of salicylate via catechol (salicylate
1-hydrolase) or via gentisate (salicylate 5-hydrolase) pathway has not been clearly
understood.
Strain XLDN2–5 (Sphingomonas sp.), a carbazole utilizing bacteria isolated from
petroleum-contaminated soil utilizes carbazole as a sole source of carbon, nitrogen,
and energy and can degrade DF and dibenzothiophene. XLDN2–5 utilizes angular
dioxygenation pathway to degrade DF to salicylic acid via an intermediate 2hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid. Also, strain XLDN2–5
degrades DT through ring cleavage and sulphoxidation pathways. This strain can
co-metabolically degrade both DF and DT using carbazole as a substrate. When
strain XLDN2–5 was incubated for 40 hr. with DT and DF; DT, DF and CA were
degraded about 90%, 100% and 100%, respectively suggesting its important role in
the bioremediation of environment/surrounding contaminated by these compounds
(Gai et al. 2007).
Furthermore, strain JB (Comamonas sp.) can co-metabolically degrade DF and
dibenzothiophene. Naphthalene is the primary substrate. It was observed that DF
was degraded to 1,2-dihydroxydibenzofuran then to 2-hydroxy-4-(3
0 -oxo-3’Hbrnzofuran-2
0 -yliden)but-2-enoic acid that was finally degraded to catechol. During
the degradation several naphthalene degrading enzymes were detected (Ji et al.
2017).
Strain RW1 (Sphingomonas sp.) and strain JWS (Burkholderia sp.) consortium
can mineralize the 4-chlorodibenzofuran via distal dioxygenation by attacking the
unsubstituted aromatic ring adjacent to ether bridge to yield 3(prm1)-chloro-2,2
(prm1),3-trihydroxybiphenyl. Further, this compound undergoes meta-cleavage to
form an intermediate that is hydrolysed to form a C-5 moiety. Later on this C-5
14 Metabolism of Dioxins and Dioxins-Like Compound, Its Regulation and. . .
299
hydrolase (2-hydroxy-6-oxo-6-phenylexa-2,4-dienate) to form salicylate and 2hydroxypenta-2,4-dienoate (Bunz and Cook 1993). This hydrolysis is same as
seen during bacterial metabolism of biphenyl (hydrolysis of 2-hydroxy-6-oxo-6phenyl-hexa-2,4-dienoate). However, the ring-cleavage product 6-(2hydroxyphenyl)ester of 2-hydroxymuconic acid formed from 2,2
0 ,3trihydroxydiphenyl ether undergoes hydrolysis to form 2-hydroxymuconate and
catechol (Wittich et al. 1992).
Thus, hydrolysis of DF and dioxin by enzymes form salicylate and 2hydroxypenta-2,4-dienoate or catechol and 2-hydroxymuconate. During degradation, either meta-cleavage pathway or ortho-cleavage pathway of catechol is
induced. Meta-cleavage pathway of catechol induced during degradation of DF
and dibenzo-p-dioxin DD by RW1 forms two intermediates 2-hydroxymuconate
and 2- hydroxypenta-2, 4-dienoate (Whitman et al. 1992; Lian and Chapman 1993).
On the other hand, ortho-cleavage of catechol is induced during DD degradation.
Thus, the degradation of catechol can be accomplished by either meta-cleavage or
ortho-cleavage; however, there is paucity of information about relative importance
of these routes. During DF degradation, catechol meta-cleavage and gentisate
pathways are induced. However, degradation of salicylate via catechol (salicylate
1-hydrolase) or via gentisate (salicylate 5-hydrolase) pathway has not been clearly
understood.
Strain XLDN2–5 (Sphingomonas sp.), a carbazole utilizing bacteria isolated from
petroleum-contaminated soil utilizes carbazole as a sole source of carbon, nitrogen,
and energy and can degrade DF and dibenzothiophene. XLDN2–5 utilizes angular
dioxygenation pathway to degrade DF to salicylic acid via an intermediate 2hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid. Also, strain XLDN2–5
degrades DT through ring cleavage and sulphoxidation pathways. This strain can
co-metabolically degrade both DF and DT using carbazole as a substrate. When
strain XLDN2–5 was incubated for 40 hr. with DT and DF; DT, DF and CA were
degraded about 90%, 100% and 100%, respectively suggesting its important role in
the bioremediation of environment/surrounding contaminated by these compounds
(Gai et al. 2007).
Furthermore, strain JB (Comamonas sp.) can co-metabolically degrade DF and
dibenzothiophene. Naphthalene is the primary substrate. It was observed that DF
was degraded to 1,2-dihydroxydibenzofuran then to 2-hydroxy-4-(3
0 -oxo-3’Hbrnzofuran-2
0 -yliden)but-2-enoic acid that was finally degraded to catechol. During
the degradation several naphthalene degrading enzymes were detected (Ji et al.
2017).
Strain RW1 (Sphingomonas sp.) and strain JWS (Burkholderia sp.) consortium
can mineralize the 4-chlorodibenzofuran via distal dioxygenation by attacking the
unsubstituted aromatic ring adjacent to ether bridge to yield 3(prm1)-chloro-2,2
(prm1),3-trihydroxybiphenyl. Further, this compound undergoes meta-cleavage to
form an intermediate that is hydrolysed to form a C-5 moiety. Later on this C-5
14 Metabolism of Dioxins and Dioxins-Like Compound, Its Regulation and. . .
299