Land plants are well-known producers of terpenoids that play diverse roles in plant–environment interactions. The vast chemical diversity of terpenoids is initiated by terpene synthases. Plants contain a distinct mid-sized terpene synthase gene family termed TPS, which appears to have an ancient origin in a fused bacterial Class I (di)terpene synthase (TS) and Class II diterpene cyclase (DTC), corresponding to the catalytically relevant α-domain and βγ-didomains, respectively. However, while such fused tridomain bifunctional (Class I/II) diterpene cyclases/synthases (DCSs) have been found in plants (and fungi), no examples have been reported from bacteria, leaving the origin of the fusion event initiating the TPS gene family opaque. Here, the discovery of such tridomain bifunctional DCSs in bacteria is reported. Extensive genome mining unearthed five putative bacterial DCSs, with biochemical characterization revealing the expected bifunctional activity for three. The most intriguing was CseDCS from Candidatus sericytochromatia bacterium, which produces ent-kaurene, an intermediate in plant hormone biosynthesis, as this is the hypothesized activity for the ancestral TPS. Unlike the extant functionally equivalent TPSs, it was possible to split CseDCS into separate, independently acting DTC and TS, with the first producing the expected ent-copalyl diphosphate (CPP), serving as a CPP synthase (CPS), while the second converts this to ent-kaurene, serving as a kaurene synthase (KS). Nevertheless, sequence alignment and mutation analysis revealed intriguing similarities between this cyanobacterial fused CPS–KS and functionally equivalent TPSs. Regardless of the exact relationship, the discovery of fused bifunctional DCSs in bacteria supports the hypothesized origin of the plant TPS family from such a bacterial gene.

中文翻译：

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在细菌中发现双功能二萜环化酶/合酶支持植物萜烯合酶基因家族的细菌起源


陆地植物是众所周知的萜类化合物生产者，萜类化合物在植物-环境相互作用中发挥着多种作用。萜类化合物的巨大化学多样性是由萜烯合酶引发的。植物包含一个独特的中等大小的萜烯合酶基因家族，称为 TPS，它似乎起源于融合细菌 I 类（二）萜烯合酶 （TS） 和 II 类二萜环化酶 （DTC），分别对应于催化相关的 α 结构域和 βγ 二结构域。然而，虽然在植物（和真菌）中发现了这种融合的三结构域双功能 （I/II 类） 二萜环化酶/合酶 （DCS），但尚未从细菌中报告任何实例，这使得启动 TPS 基因家族的融合事件的起源不透明。在这里，报道了在细菌中发现这种三结构域双功能 DCS。广泛的基因组挖掘发现了 5 种推定的细菌 DCS，生化表征揭示了 3 种的预期双功能活性。最有趣的是来自丝网珠菌 CseDCS，它产生植物激素生物合成的中间体 ent-kaurene，因为这是祖先 TPS 的假设活性。与现存的功能等效的 TPS 不同，可以将 CseDCS 分裂成单独的、独立作用的 DTC 和 TS，第一个产生预期的 ent-copalyl diphosphate （CPP），作为 CPP 合酶 （CPS），而第二个将其转化为 ent-kaurene，作为 kaurene 合酶 （KS）。尽管如此，序列比对和突变分析揭示了这种蓝藻融合的 CPS-KS 和功能等效的 TPS 之间的有趣相似之处。 无论确切的关系如何，在细菌中发现融合的双功能 DCS 都支持植物 TPS 家族起源于这种细菌基因的假设。