Department of Plant Pathology, University of California, Davis, California, USA.
Department of Plant Pathology, University of California, Davis, California, USA
Appl Environ Microbiol. 2018 Sep 17;84(19). doi: 10.1128/AEM.01057-18. Print 2018 Oct 1.
We show for soil bacterium LF7 that the possession of an ndole-3-cetic acid atabolic () gene cluster is causatively linked to the ability to utilize the plant hormone indole-3-acetic acid (IAA) as a carbon and energy source. Genome-wide transcriptional profiling by mRNA sequencing revealed that these genes, chromosomally arranged as and coding for the transformation of IAA to catechol, were the most highly induced (>29-fold) among the relatively few (<1%) differentially expressed genes in response to IAA. Also highly induced and immediately downstream of the cluster were genes for a major facilitator superfamily protein () and enzymes of the β-ketoadipate pathway (), which channels catechol into central metabolism. This entire gene set was constitutively expressed in an deletion mutant, confirming the role of , annotated as coding for a MarR-type regulator and located upstream of , as a repressor of gene expression. In LF7 carrying the DNA region upstream of fused to a promoterless gene, green fluorescence accumulated in response to IAA at concentrations as low as 1.6 μM. The promoter region also responded to chlorinated IAA, but not other aromatics tested, indicating a narrow substrate specificity. In an deletion mutant, expression from the promoter region was constitutive, consistent with the predicted role of as a repressor. A deletion analysis revealed putative -35/-10 promoter sequences upstream of , as well as a possible binding site for the IacR repressor. Bacterial genes code for the enzymatic conversion of the plant hormone indole-3-acetic acid (IAA) to catechol. Here, we demonstrate that the genes of soil bacterium LF7 enable growth on IAA by coarrangement and coexpression with a set of and genes that code for complete conversion of catechol to central metabolites. This work contributes in a number of novel and significant ways to our understanding of gene biology in bacteria from (non-)plant environments. More specifically, we show that LF7's response to IAA involves derepression of the MarR-type transcriptional regulator IacR, which is quite fast (less than 25 min upon IAA exposure), highly specific (only in response to IAA and chlorinated IAA, and with few genes other than , , and induced), relatively sensitive (low micromolar range), and seemingly tailored to exploit IAA as a source of carbon and energy.
我们证明，土壤细菌 LF7 拥有一个吲哚-3-乙酸代谢（indole-3-acetic acid metabolism）基因簇，这使得它能够将植物激素吲哚-3-乙酸（indole-3-acetic acid，IAA）作为碳源和能源加以利用。通过 mRNA 测序的全基因组转录谱分析表明，这些基因在相对较少（<1%）的差异表达基因中，被高度诱导（>29 倍），这些基因在染色体上排列为 ，并编码将 IAA 转化为儿茶酚的过程。在 基因簇的下游，还有编码主要易化因子超级家族蛋白（major facilitator superfamily protein）和β-酮戊二酸途径（β-ketoadipate pathway）酶的基因被高度诱导，β-酮戊二酸途径将儿茶酚导入中心代谢途径。这个由 个基因组成的整套基因在 缺失突变体中持续表达，证实了 基因（被注释为编码 MarR 型调控蛋白）和位于 上游的基因（）作为 基因表达的阻遏物的作用。在携带融合到无启动子的 基因上游 DNA 区域的 LF7 中，在低至 1.6 μM 的 IAA 浓度下，绿色荧光就会累积。 启动子区域也对氯化 IAA 有反应，但对其他测试的芳香族化合物没有反应，表明其底物特异性较窄。在 缺失突变体中， 基因从 启动子区域的表达是组成型的，这与 作为阻遏物的预期作用一致。缺失分析显示，在 上游存在可能的 -35/-10 启动子序列，以及可能与 IacR 阻遏物结合的位点。细菌 基因编码将植物激素吲哚-3-乙酸（IAA）转化为儿茶酚的酶。在这里，我们证明了土壤细菌 LF7 的 基因通过与一组编码将儿茶酚完全转化为中心代谢物的 和 基因的共排列和共表达，使其能够利用 IAA 进行生长。这项工作在许多新的和重要的方面促进了我们对非植物环境中细菌 基因生物学的理解。更具体地说，我们表明，LF7 对 IAA 的反应涉及 MarR 型转录调控因子 IacR 的去阻遏，这一过程非常迅速（在接触 IAA 后不到 25 分钟），高度特异性（仅对 IAA 和氯化 IAA 有反应，除了 、 和 基因之外，其他基因的诱导很少），相对敏感（低微摩尔范围），并且似乎专门用于利用 IAA 作为碳源和能源。
