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|Section2= |Section3= }} Momilactone B is an allelopathic agent produced from the roots of rice (''Oryza sativa'' L.) (100 mg from 200 kg dry rice husk). It has been shown to be produced in high concentrations by the roots of rice seedlings. The production of momilactone B has also been induced in response to infection by blast fungus (''Pyricularia oryzae'') or irradiated with UV light. More recently it has been shown to be a potential chemotherapeutic agent against human colon cancer. ==Biosynthesis== Of the natural products isolated from rice, fifteen have been shown to be phytoalexins. Of these fifteen, fourteen, including momilactone B, are diterpenes commonly biosynthesized from geranylgeranyl diphosphate (GGDP). The first step in the biosynthesis of momilactone B is the cyclization of GGDP to ''syn''-copalyl diphosphate (''syn''-CDP), a type B cyclization. This is initiated by the addition of a proton to the terminal olefin bond of GGDP. The genes encoding for ''syn''-CDP synthase was found by Otomo ''et al.''. in 2004. This was labeled as ''OsCyc''1 (''Oryza sativa Cyclase'' 1). According to the rice genome database, ''OsCyc1'' corresponds to ''OsCPS''4 (AL662933) found in chromosome 4 (14.3cM) discovered by Sakamoto ''et al.''. in 2004. The second step is the cyclization of ''syn''-CDP to 9β-pimara-7,15-diene. This step is initiated by the elimination of the diphosphate group, a type A cyclization. The genes encoding for the type A cyclase were found by Otomo ''et al.''. in 2004. It is suggested that ''OsKS''4, located on chromosome 4 (14.3cM) is one of the genes responsible for phytoalexin biosynthesis. After UV-radiation, ''OsKS''4 mRNA levels rise drastically in response to the attack. Further oxidation of the 9β-pimara-7,15-diene compound followed by lactone and lactol formation yields momilactone B. Though the gene sequence of these oxidations is unknown, P450 monooxygenase genes have been identified near the cyclase genes on chromosome 4. This suggests that the complete gene cluster for the biosynthesis of momilactone B is located on chromosome 4 of rice. Further research is being done on isolating the enzymes responsible for the biosynthesis of momilactone B and other phytoalexins found in rice. Once these enzymes are found, the stereochemistry of these reactions could be examined. Further studies into the biosynthesis of momilactone B could prove useful in studying the defense mechanisms of rice. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Momilactone B」の詳細全文を読む スポンサード リンク
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