ABSTRACT
In the aflatoxin biosynthetic pathway, 5′-oxoaverantin (OAVN) cyclase, the cytosolic enzyme, catalyzes the reaction from OAVN to (2′
S
,5′
S
)-averufin (AVR) (E. Sakuno, K. Yabe, and H. Nakajima, Appl. Environ. Microbiol.
69:
6418-6426, 2003). Interestingly, the
N
-terminal 25-amino-acid sequence of OAVN cyclase completely matched an internal sequence of the versiconal (VHOH) cyclase that was deduced from its gene (
vbs
). The purified OAVN cyclase also catalyzed the reaction from VHOH to versicolorin B (VB). In a competition experiment using the cytosol fraction of
Aspergillus parasiticus
, a high concentration of VHOH inhibited the enzyme reaction from OAVN to AVR, and instead VB was newly formed. The recombinant Vbs protein, which was expressed in
Pichia pastoris
, showed OAVN cyclase activity, as well as VHOH cyclase activity. A mutant of
A. parasiticus
SYS-4 (= NRRL 2999) with
vbs
deleted accumulated large amounts of OAVN, 5′-hydroxyaverantin, averantin, AVR, and averufanin in the mycelium. These results indicated that the cyclase encoded by the
vbs
gene is also involved in the reaction from OAVN to AVR in aflatoxin biosynthesis. Small amounts of VHOH, VB, and aflatoxins also accumulated in the same mutant, and this accumulation may have been due to an unknown enzyme(s) not involved in aflatoxin biosynthesis. This is the first report of one enzyme catalyzing two different reactions in a pathway of secondary metabolism.
摘要
在黄曲霉毒素生物合成途径中,5′-氧代藜芦素(OAVN)环化酶(细胞膜酶)催化从OAVN到(2′-S)的反应。
S
,5′
S
)-狸花素(AVR)的反应(E. Sakuno, K. Yabe, and H. Nakajima, Appl.Microbiol.
69:
6418-6426, 2003).有趣的是
N
-末端 25-amino-acid 序列完全符合从其基因中推导出的 versiconal(VHOH)环化酶的内部序列 (
vbs
).纯化的 OAVN 环化酶还能催化从 VHOH 到 versicolorin B(VB)的反应。在使用寄生曲霉的细胞质部分进行的竞争实验中
寄生曲霉
的竞争实验中,高浓度的 VHOH 抑制了从 OAVN 到 AVR 的酶反应,取而代之的是 VB 的新生成。重组的 Vbs 蛋白在
Pichia pastoris
中表达的重组 Vbs 蛋白具有 OAVN 环化酶活性和 VHOH 环化酶活性。一种
寄生虫
SYS-4(= NRRL 2999)的突变体具有
vbs
删除的突变体在菌丝体中积累了大量的 OAVN、5′-羟基阿维菌素、阿维菌素、AVR 和阿维菌素。这些结果表明,由
vbs
基因编码的环化酶也参与了黄曲霉毒素生物合成过程中从 OAVN 到 AVR 的反应。同一突变体中也积累了少量的 VHOH、VB 和黄曲霉毒素,这种积累可能是由于一种或多种不参与黄曲霉毒素生物合成的未知酶造成的。这是首次报道一种酶在次级代谢途径中催化两种不同的反应。