Regulatory cascade and biological activity of Beauveria bassiana oosporein that limits bacterial growth after host death
Regulatory cascade and biological activity of Beauveria bassiana oosporein that limits bacterial growth after host death
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白僵菌卵孢子蛋白的调控级联和生物活性限制宿主死亡后细菌的生长
DOI：
10.1073/pnas.1616543114
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发表时间：
2017-02-28
影响因子：
11.1
通讯作者：
Tong, Sheng
中科院分区：
文献类型：
--
作者：
Fan, Yanhua;Liu, Xi;Tong, Sheng
自70多年前发现卵孢子素以来，关于其潜在的抗菌和杀虫活性的报道一直相互矛盾。我们的研究结果表明，卵孢子蛋白不太可能作为昆虫毒素或参与早期至中期感染过程，包括渗透和免疫逃避。相反，卵孢子素最有可能在宿主死亡后发挥作用，以阻止宿主尸体上的细菌竞争，使真菌能够最大限度地利用宿主营养物质并完成其生命周期。我们的数据还表明，卵孢子素生产是由一系列转录因子调控的，BbSmr 1作为上游负调控因子，靶向OpS 3的表达，OpS 3又作为卵孢子素生物合成基因簇的正调控因子。真菌次级代谢产物卵孢子素的调控网络和生物学功能仍然不清楚。球孢白僵菌已经进化出寄生昆虫和战胜微生物挑战者同化宿主营养物质的能力。BbSmr 1（B. bassiana次级代谢物调节剂1），在卵孢子素过量产生的筛选中被鉴定。Bbsmr 1的缺失导致卵孢子素生物合成基因簇（OpS基因）和组成型卵孢子素生产的上调。在卵孢子蛋白基因簇中，Bbsmr 1和第二个转录因子OpS 3的双突变体（Δ Bbsmr 1 Δ OpS 3）中卵孢子蛋白的产生被消除，表明Bbsmr 1作为OpS 3表达的负调节因子。实时定量PCR和GFP启动子融合构建的卵孢子素聚酮合酶OpS 1，表明OpS 1主要在昆虫尸体死亡后24-48 h表达。B中的细菌菌落分析。在宿主死亡之前，白僵菌感染的昆虫宿主的数量一直在增加，死亡后数量急剧下降（约90%），这与卵孢子素的产生有关。体外研究证实卵孢子素对来自昆虫尸体的细菌的抑制活性。这些结果表明，卵孢子蛋白作为一种抗微生物化合物，以限制微生物对B的竞争。这使得真菌能够最大限度地利用宿主的营养物质在受感染的尸体上生长和形成孢子。
Significance Since the discovery of oosporein more than 70 years ago, there have been conflicting reports on its potential antimicrobial and insecticidal activities. Our results indicate that oosporein is unlikely to function as an insect toxin or to be involved in early to mid-infection processes, including penetration and immune evasion. Instead, oosporein most likely functions after death of the host to thwart bacterial competition on a host cadaver, allowing the fungus to maximally use host nutrients and complete its life cycle. Our data also reveal that oosporein production is regulated by a cascade of transcription factors, with BbSmr1 acting as an upstream negative regulator, targeting the expression of OpS3, which in turn acts as a positive regulator of the oosporein biosynthetic gene cluster. The regulatory network and biological functions of the fungal secondary metabolite oosporein have remained obscure. Beauveria bassiana has evolved the ability to parasitize insects and outcompete microbial challengers for assimilation of host nutrients. A novel zinc finger transcription factor, BbSmr1 (B. bassiana secondary metabolite regulator 1), was identified in a screen for oosporein overproduction. Deletion of Bbsmr1 resulted in up-regulation of the oosporein biosynthetic gene cluster (OpS genes) and constitutive oosporein production. Oosporein production was abolished in double mutants of Bbsmr1 and a second transcription factor, OpS3, within the oosporein gene cluster (ΔBbsmr1ΔOpS3), indicating that BbSmr1 acts as a negative regulator of OpS3 expression. Real-time quantitative PCR and a GFP promoter fusion construct of OpS1, the oosporein polyketide synthase, indicated that OpS1 is expressed mainly in insect cadavers at 24–48 h after death. Bacterial colony analysis in B. bassiana-infected insect hosts revealed increasing counts until host death, with a dramatic decrease (∼90%) after death that correlated with oosporein production. In vitro studies verified the inhibitory activity of oosporein against bacteria derived from insect cadavers. These results suggest that oosporein acts as an antimicrobial compound to limit microbial competition on B. bassiana-killed hosts, allowing the fungus to maximally use host nutrients to grow and sporulate on infected cadavers.