CBC轉錄因子在不同環境刺激下調控白色念珠菌致病因子之探討

CCAAT為普遍存在於真核生物基因體之中的DNA序列。生物體中負責辨識並結合在CCAAT序列上的蛋白複合體被命名為「CCAAT-binding complex」，簡稱為CBC。CBC的胺基酸序列在演化上高度保存於真菌、植物及哺乳類動物之中。CBC可自行調節基因轉錄，亦可藉由與其它轉錄因子共同合作來進行轉錄調控。Hap43蛋白是在白色念珠菌的研究中第一個被鑑定為可跟CBC結合並共同作用的轉錄因子。本研究中先進行Hap43的功能分析。結果初步確認Hap43是一個在缺鐵環境中會被活化的抑制型轉錄因子，專門負責在鐵離子不足的情況下壓制「需鐵類型蛋白」之基因的轉錄。另外，缺鐵環境可促使Hap43聚集在細胞核內，同時Hap43調控基因表達的功能需仰賴CBC的存在。然而，迄今在白色念珠菌的研究中，CBC是否具有不需與Hap43協同作用的功能仍不清楚。因此本研究的第二部份著重於CBC總體的功能分析，發現了CBC負責調控許多白色念珠菌的致病因子。另外，藉由比較正常菌株及CBC剃除菌株的全基因體轉錄變化情形，推測CBC亦負責「核醣體生合成」和「轉譯作用」的負向調控，以及「缺氮反應」的正向調控。同時CBC還負責調控數種重要的表現型，包括「低氮環境激發的菌絲生成」、「使用蛋白質分解後獲得之氮源的能力」，及「抑制酵母菌細胞凝集作用」。有趣的是，上位基因效應分析的結果推測出CBC隸屬於Rhb1-TOR訊息傳遞鏈，同時CBC可利用Mep2-Ras1-PKA/MAPK的訊息傳遞途徑來控制「低氮環境激發的菌絲生成」。最後，本研究將HAP43及CBC基因突變後，發現會降低白色念珠菌的致病力。總結來說，本研究發掘CBC蛋白複合體在接受環境變化刺激後調控白色念珠菌致病因子之重要角色。這些發現不僅有助於了解白色念珠菌，同時還可能被借鏡到其它人類致病真菌的病理研究。

關鍵字

白色念珠菌； CCAAT序列結合蛋白；致病因子；鐵平衡調控；菌絲生成； Rapamycin標靶蛋白訊息傳遞鏈

並列摘要

The CCAAT-motif is ubiquitous in promoters of eukaryotic genomes. The CCAAT-binding complex (CBC), which is conserved from fungi to plants, to mammals, specifically recognizes the CCAAT-motif and modulates transcription directly or in cooperation with other transcription factors. In Candida albicans, Hap43 was identified to interact with CBC. In this study, the function of Hap43 was first characterized. The results demonstrated that Hap43 is a low iron–induced transcription factor and is responsible for repression of genes encoding iron-dependent proteins in response to iron deprivation. Iron deprived conditions can induce the nuclear localization of Hap43. Moreover, Hap43-mediated regulation was shown to depend on the presence of CBC. However, it is unclear whether CBC can function independently of Hap43. The Hap43-independent role of CBC in C. albicans was further explored and the results showed that CBC acts as a novel regulator of virulence traits. Genome-wide transcriptional profiling suggested that CBC contributes to negative regulation of ribosome biogenesis and translation and positive regulation of nitrogen starvation-mediated responses. CBC was also required for low nitrogen–induced filamentation, utilization of nitrogen sources from proteins, and repression of flocculation phenotype. Interestingly, epistasis analyses suggested that CBC is an important downstream effector of Rhb1-TOR signaling and controls low nitrogen–induced filamentation via the Mep2-Ras1-PKA/MAPK pathway. Finally, deletion of HAP43 and CBC genes attenuated C. albicans virulence. This study therefore highlights the crucial role of C. albicans CBC in regulating multiple virulence traits in response to environmental perturbations. These findings not only provide basic information for C. albicans per se but also may increase our understanding of the pathogenesis of other human fungal pathogens.

並列關鍵字

Candida albicans ； CCAAT-binding complex ； Virulence factor ； Iron homeostasis ； Filamentation ； TOR signaling

參考文獻

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CBC轉錄因子在不同環境刺激下調控白色念珠菌致病因子之探討

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