Biotechnology Research Center, The University of Tokyo, Tokyo, Japan
Department of Microbiology, RK University, Rajkot, Gujarat, India.
Appl Environ Microbiol. 2021 May 11;87(11). doi: 10.1128/AEM.01589-20.
A novel gene cluster involved in the degradation of lignin-derived monoaromatics such as -hydroxybenzoate, vanillate, and ferulate has been identified in the thermophilic nitrate reducer JL-2. Based on conserved domain analyses and metabolic pathway mapping, the cluster was classified into upper- and peripheral-pathway operons. The upper-pathway genes, responsible for the degradation of -hydroxybenzoate and vanillate, are located on a 0.27-Mb plasmid, whereas the peripheral-pathway genes, responsible for the transformation of ferulate, are spread throughout the plasmid and the chromosome. In addition, a lower-pathway operon was also identified in the plasmid that corresponds to the cleavage pathway of catechol. Spectrophotometric and gene induction data suggest that the upper and lower operons are induced by -hydroxybenzoate, which the strain can degrade completely within 4 days of incubation, whereas the peripheral genes are expressed constitutively. The upper degradation pathway follows a less common route, proceeding via the decarboxylation of protocatechuate to form catechol, and involves a novel thermostable γ-carboxymuconolactone decarboxylase homolog, identified as protocatechuate decarboxylase based on gene deletion experiments. This gene cluster is conserved in only a few members of the and shows traces of vertical expansion of catabolic pathways in these organisms toward lignoaromatics. High-temperature steam treatment of lignocellulosic biomass during the extraction of cellulose and hemicellulose fractions leads to the release of a wide array of lignin-derived aromatics into the natural ecosystem, some of which can have detrimental effects on the environment. Not only will identifying organisms capable of using such aromatics aid in environmental cleanup, but thermostable enzymes, if characterized, can also be used for efficient lignin valorization. However, no thermophilic lignin degraders have been reported thus far. The present study reports JL-2 as a thermophilic bacterium with the potential to use lignin-derived aromatics. The identification of a novel thermostable protocatechuate decarboxylase gene in the strain further adds to its significance, as such an enzyme can be efficiently used in the biosynthesis of ,-muconate, an important intermediate in the commercial production of plastics.
一个新型基因簇被鉴定参与木质素衍生单芳族化合物的降解，如 - 羟基苯甲酸、香草酸和阿魏酸，该基因簇存在于嗜热硝酸盐还原菌 JL-2 中。根据保守结构域分析和代谢途径映射，该基因簇被分类为上途径和外围途径操纵子。上途径基因负责 - 羟基苯甲酸和香草酸的降解，位于一个 0.27-Mb 的质粒上，而外围途径基因，负责阿魏酸的转化，分布在质粒和染色体上。此外，在质粒中还鉴定出一个下途径操纵子，对应儿茶酚的裂解途径。分光光度法和基因诱导数据表明，上和下途径操纵子被 - 羟基苯甲酸诱导，该菌株在孵育 4 天内可完全降解 - 羟基苯甲酸，而外围基因则持续表达。上降解途径遵循一种不太常见的途径，通过原儿茶酸的脱羧作用形成儿茶酚，并涉及一种新型耐热γ-羧基戊二酸内酯脱羧酶同源物，根据基因缺失实验鉴定为原儿茶酸脱羧酶。该基因簇仅在少数 和 属的成员中保守，并显示出这些生物体中木质素芳香族化合物代谢途径向木质素芳香族化合物的垂直扩展的痕迹。在纤维素和半纤维素部分提取过程中，高温蒸汽处理木质纤维素生物质会导致大量木质素衍生芳香族化合物释放到自然生态系统中，其中一些对环境可能有害。不仅鉴定出能够利用此类芳香族化合物的生物有助于环境清理，而且如果表征耐热酶，也可以用于有效的木质素增值。然而，迄今为止尚未报道耐热木质素降解菌。本研究报告 JL-2 是一种具有利用木质素衍生芳香族化合物潜力的嗜热细菌。该菌株中新型耐热原儿茶酸脱羧酶基因的鉴定进一步增加了其意义，因为这种酶可以有效地用于 ，- 戊二酸的生物合成，这是塑料商业生产中的一个重要中间体。
