Motif discovery in upregulated genes.

Saccharomycopsis yeasts are natural organic sulfur auxotrophs due to lack of genes required for the uptake and assimilation of sulfate/sulfite. Starvation for methionine induces a shift to a predatory, mycoparasitic life strategy that is unique amongst ascomycetous yeasts. Similar to fungal plant pathogens that separated from Saccharomycopsis more than 400 million years ago, a specialized infection structure called penetration peg is used for prey cell invasion. Penetration pegs are highly enriched with chitin. Here we demonstrate that an ancient and conserved MAP kinase signaling pathway regulates penetration peg formation and successful predation in the predator yeast S. schoenii. Deletion of the MAP kinase gene SsKIL1, a homolog of the Saccharomyces cerevisiae ScKSS1/ScFUS3 and the rice blast Magnaporthe oryzae MoPMK1 genes, as well as deletion of the transcription factor SsSTE12 generate non-pathogenic mutants that fail to form penetration pegs. Comparative global transcriptome analyses using RNAseq indicate loss of the SsKil1-SsSte12-dependent predation response in the mutant strains, while a methionine starvation response is still executed. Within the promoter sequences of genes upregulated during predation we identified a cis-regulatory element similar to the ScSte12 pheromone response element. Our results indicate that, re-routing MAP-kinase signaling by re-wiring Ste12 transcriptional control towards predation specific genes contributed to the parallel evolution of this predacious behaviour in predator yeasts. Consequently, we found that SsSTE12 is dispensable for mating.

扣囊酵母属（Saccharomycopsis）酵母因缺乏摄取与同化硫酸盐/亚硫酸盐的相关基因，属于天然有机硫营养缺陷型菌株。甲硫氨酸饥饿会诱导其转向捕食性、真菌寄生的生活策略，该特性在子囊菌酵母中独树一帜。与在4亿多年前与扣囊酵母属分化的植物病原真菌类似，该类酵母可借助一种名为侵入钉（penetration peg）的特化侵染结构侵入猎物细胞。侵入钉富含几丁质（chitin）。本研究证实，在捕食性酵母S. schoenii中，一条古老且保守的丝裂原活化蛋白激酶（MAP kinase, MAPK）信号通路可调控侵入钉的形成与成功捕食行为。敲除丝裂原活化蛋白激酶基因SsKIL1（其为酿酒酵母（Saccharomyces cerevisiae）ScKSS1/ScFUS3与稻瘟病菌（Magnaporthe oryzae）MoPMK1的同源基因），以及敲除转录因子SsSTE12，均会产生无法形成侵入钉的非致病性突变体。利用RNA测序（RNAseq）开展的全转录组比较分析显示，突变株中依赖SsKil1-SsSte12的捕食响应通路出现缺失，但甲硫氨酸饥饿响应仍可正常执行。在捕食过程中上调基因的启动子序列内，我们鉴定出了一个与酿酒酵母ScSte12信息素响应元件相似的顺式调控元件。本研究结果表明，通过将Ste12的转录调控重定向至捕食特异性基因，进而重编程丝裂原活化蛋白激酶信号通路，这一机制推动了捕食性酵母中该捕食行为的平行演化。此外，我们发现SsSTE12对于酵母的交配过程并非必需。