Habib Cameron, Yu Yiyang, Gozzi Kevin, Ching Carly, Shemesh Moshe, Chai Yunrong
Department of Biology, Northeastern University, Boston, MA, United States of America.
Agricultural Research Organization The Volcani Center, Rishon LeZion, Israel.
PLoS One. 2017 Jun 15;12(6):e0179761. doi: 10.1371/journal.pone.0179761. eCollection 2017.
The soil bacterium Bacillus subtilis is often found in association with plants in the rhizosphere. Previously, plant polysaccharides have been shown to stimulate formation of root-associated multicellular communities, or biofilms, in this bacterium, yet the underlying mechanism is not fully understood. A five-gene gan operon (ganSPQAB) in B. subtilis has recently been shown to be involved in utilization of the plant-derived polysaccharide galactan. Despite these findings, molecular details about the regulation of the operon and the role of the operon in biofilm formation remain elusive. In this study, we performed comprehensive genetic analyses on the regulation of the gan operon. We show that this operon is regulated both by a LacI-like transcription repressor (GanR), which directly binds to pairs of inverted DNA repeats in the promoter region of the operon, and by the catabolite control protein A (CcpA). Derepression can be triggered by the presence of the inducer β-1,4-galactobiose, a hydrolysis product of galactan, or in situ when B. subtilis cells are associated with plant roots. In addition to the transcriptional regulation, the encoded ß-galactosidase GanA (by ganA), which hydrolyzes ß-1,4-galactobiose into galactose, is inhibited at the enzymatic level by the catalytic product galactose. Thus, the galactan utilization pathway is under complex regulation involving both positive and negative feedback mechanisms in B. subtilis. We discuss about the biological significance of such complex regulation as well as a hypothesis of biofilm induction by galactan via multiple mechanisms.
土壤细菌枯草芽孢杆菌经常在根际与植物共生。此前已表明，植物多糖可刺激该细菌形成与根相关的多细胞群落或生物膜，但其潜在机制尚未完全了解。枯草芽孢杆菌中的一个五基因gan操纵子（ganSPQAB）最近被证明参与植物衍生多糖半乳聚糖的利用。尽管有这些发现，但关于该操纵子调控的分子细节以及该操纵子在生物膜形成中的作用仍然不清楚。在本研究中，我们对gan操纵子的调控进行了全面的遗传分析。我们表明，该操纵子既受一种类似LacI的转录阻遏物（GanR）调控，GanR直接结合到操纵子启动子区域的反向DNA重复序列对，也受分解代谢物控制蛋白A（CcpA）调控。诱导剂β-1,4-半乳糖二糖（半乳聚糖的水解产物）的存在，或枯草芽孢杆菌细胞与植物根相关时的原位情况，均可触发去阻遏。除了转录调控外，编码的β-半乳糖苷酶GanA（由ganA编码）将β-1,4-半乳糖二糖水解为半乳糖，在酶水平上受到催化产物半乳糖的抑制。因此，在枯草芽孢杆菌中，半乳聚糖利用途径受到复杂调控，涉及正负反馈机制。我们讨论了这种复杂调控的生物学意义以及半乳聚糖通过多种机制诱导生物膜形成的假说。
