ABSTRACT
Pseudomonas
sp. strain U2 was isolated from oil-contaminated soil in Venezuela by selective enrichment on naphthalene as the sole carbon source. The genes for naphthalene dioxygenase were cloned from the plasmid DNA of strain U2 on an 8.3-kb
Bam
HI fragment. The genes for the naphthalene dioxygenase genes
nagAa
(for ferredoxin reductase),
nagAb
(for ferredoxin), and
nagAc
and
nagAd
(for the large and small subunits of dioxygenase, respectively) were located by Southern hybridizations and by nucleotide sequencing. The genes for
nagB
(for naphthalene
cis
-dihydrodiol dehydrogenase) and
nagF
(for salicylaldehyde dehydrogenase) were inferred from subclones by their biochemical activities. Between
nagAa
and
nagAb
were two open reading frames, homologs of which have also been identified in similar locations in two nitrotoluene-using strains (J. V. Parales, A. Kumar, R. E. Parales, and D. T. Gibson, Gene 181:57–61, 1996; W.-C. Suen, B. Haigler, and J. C. Spain, J. Bacteriol. 178:4926–4934, 1996) and a naphthalene-using strain (G. J. Zylstra, E. Kim, and A. K. Goyal, Genet. Eng. 19:257–269, 1997). Recombinant
Escherichia coli
strains with plasmids carrying this region were able to convert salicylate to gentisate, which was identified by a combination of gas chromatography-mass spectrometry and nuclear magnetic resonance. The first open reading frame, designated
nagG
, encodes a protein with characteristics of a Rieske-type iron-sulfur center homologous to the large subunits of dihydroxylating dioxygenases, and the second open reading frame, designated
nagH
, encodes a protein with limited homology to the small subunits of the same dioxygenases. Cloned together in
E. coli
,
nagG
,
nagH
, and
nagAb
, were able to convert salicylate (2-hydroxybenzoate) into gentisate (2,5-dihydroxybenzoate) and therefore encode a salicylate 5-hydroxylase activity. Single-gene knockouts of
nagG
,
nagH
, and
nagAb
demonstrated their functional roles in the formation of gentisate. It is proposed that NagG and NagH are structural subunits of salicylate 5-hydroxylase linked to an electron transport chain consisting of NagAb and NagAa, although
E. coli
appears to be able to partially substitute for the latter. This constitutes a novel mechanism for monohydroxylation of the aromatic ring. Salicylate hydroxylase and catechol 2,3-dioxygenase in strain U2 could not be detected either by enzyme assay or by Southern hybridization. However growth on both naphthalene and salicylate caused induction of gentisate 1,2-dioxygenase, confirming this route for salicylate catabolism in strain U2. Sequence comparisons suggest that the novel gene order
nagAa-nagG-nagH-nagAb-nagAc-nagAd-nagB-nagF
represents the archetype for naphthalene strains which use the gentisate pathway rather than the
meta
cleavage pathway of catechol.
摘要
假单胞菌
菌株 U2 是通过选择性富集萘作为唯一碳源从委内瑞拉受石油污染的土壤中分离出来的。从菌株 U2 的质粒 DNA 中克隆出了萘二氧合酶的基因,该基因被克隆在一个 8.3-kb
Bam
HI 片段。萘二氧化酶的基因为
nagAa
(用于铁氧还原酶)、
nagAb
(铁氧还原酶)和
nagAc
和
和
(分别代表二氧化酶的大亚基和小亚基)的基因。nagB
nagB
(的基因
顺
-二氢二醇脱氢酶)和
和
(水杨醛脱氢酶)是根据其生化活性从亚克隆中推断出来的。在
nagAa
和
nagAb
之间有两个开放阅读框,其同源物也已在两个使用硝基甲苯的菌株中的类似位置被发现(J. V. Parales、A. Kumar、R. E. Parales 和 D. T. Gibson,Gene 181:57-61,1996;W.-C.Suen、B. Haigler 和 J. C. Spain,J. Bacteriol.178:4926-4934, 1996)和一株使用萘的菌株(G. J. Zylstra, E. Kim, and A. K. Goyal, Genet.)重组
大肠杆菌
质粒的菌株能够将水杨酸盐转化为龙胆二酸盐,龙胆二酸盐是通过气相色谱-质谱联用仪和核磁共振鉴定出来的。第一个开放阅读框被命名为
nagG
编码的蛋白质具有与二羟化二氧酶大亚基同源的里斯克型铁硫中心的特征,第二个开放阅读框编码的蛋白质具有与二羟化二氧酶大亚基同源的里斯克型铁硫中心的特征,第二个开放阅读框编码的蛋白质具有与二羟化二氧酶大亚基同源的里斯克型铁硫中心的特征。
nagH
编码的蛋白质与同一种二氧合酶的小亚基具有有限的同源性。一起克隆到
大肠杆菌
,
nagG
,
nagH
和
nagAb
能将水杨酸盐(2-羟基苯甲酸盐)转化为庆大霉素(2,5-二羟基苯甲酸盐),因此编码水杨酸盐 5-羟化酶活性。单基因敲除
nagG
,
nagH
和
nagAb
证明了它们在形成庆大霉素中的功能作用。据推测,NagG 和 NagH 是水杨酸 5-羟化酶的结构亚基,它们与由 NagAb 和 NagAa 组成的电子传递链相连接,不过,NagAb 和 NagAa 在水杨酸 5-羟化酶的形成过程中起着重要作用。
大肠杆菌
似乎能够部分替代后者。这构成了芳香环单羟化的新机制。无论是通过酶测定还是 Southern 杂交,都无法检测到菌株 U2 中的水杨酸羟化酶和儿茶酚 2,3- 二氧化酶。然而,萘和水杨酸的生长会诱导龙胆二酸 1,2-二加氧酶,这证实了水杨酸在菌株 U2 中的分解途径。序列比较表明,新基因顺序为
nagAa-nagG-nagH-nagAb-nagAc-nagAd-nagB-nagF
代表了萘菌株的原型,它们使用庆大霉素途径而不是
元
儿茶酚的裂解途径。