The pH-Responsive Transcription Factors YlRim101 and Mhy1 Regulate Alkaline pH-Induced Filamentation in the Dimorphic Yeast Yarrowia lipolytica

Abstract

Environmental pH influences cell growth and differentiation. In the dimorphic yeast Yarrowia lipolytica, neutral-alkaline pH strongly induces the yeast-to-filament transition. However, the regulatory mechanism that governs alkaline pH-induced filamentation has been unclear. Here, we show that the pH-responsive transcription factor Y. lipolytica Rim101 (YlRim101) is a major regulator of alkaline-induced filamentation, since the deletion of YlRIM101 severely impaired filamentation at alkaline pH, whereas the constitutively active YlRIM1011-330 mutant mildly induced filamentation at acidic pH. YlRim101 controls the expression of the majority of alkaline-regulated cell wall protein genes. One of these, the cell surface glycosidase gene YlPHR1, plays a critical role in growth, cell wall function, and filamentation at alkaline pH. This finding suggests that YlRim101 promotes filamentation at alkaline pH via controlling the expression of these genes. We also show that, in addition to YlRim101, the Msn2/Msn4-like transcription factor Mhy1 is highly upregulated at alkaline pH and is essential for filamentation. However, unlike YlRim101, which specifically regulates alkaline-induced filamentation, Mhy1 regulates both alkaline- and glucose-induced filamentation, since the deletion of MHY1 abolished them both, whereas the overexpression of MHY1 induced strong filamentation irrespective of the pH or the presence of glucose. Finally, we show that YlRim101 and Mhy1 positively coregulate seven cell wall protein genes at alkaline pH, including YlPHR1 and five cell surface adhesin-like genes, three of which appear to promote filamentation. Together, these results reveal a conserved role of YlRim101 and a novel role of Mhy1 in the regulation of alkaline-induced filamentation in Y. lipolytica. IMPORTANCE The regulatory mechanism that governs pH-regulated filamentation is not clear in dimorphic fungi except in Candida albicans. Here, we investigated the regulation of alkaline pH-induced filamentation in Yarrowia lipolytica, a dimorphic yeast distantly related to C. albicans. Our results show that the transcription factor YlRim101 and the Msn2/Msn4-like transcription factor Mhy1 are the major regulators that promote filamentation at alkaline pH. They control the expression of a number of cell wall protein genes important for cell wall organization and filamentation. Our results suggest that the Rim101/PacC homologs play a conserved role in pH-regulated filamentation in dimorphic fungi.

The Zinc Finger Transcription Factor Fts2 Represses the Yeast-to-Filament Transition in the Dimorphic Yeast Yarrowia lipolytica

“…Yl PHR1 was upregulated by 12.8-fold. Among these genes, only Yl PHR1 is known to be required for filamentation ( 9 ). The roles of other major target genes in filamentation are not clear.…”

“…Of these, 7 genes except the 3 genes encoding proteins similar to S. cerevisiae Cwp1 or Tir1/Tir3 ( YALI0F07535 , YALI0E11517 , and YALI0E31108 ) were also upregulated during alkaline-induced filamentation. Six genes including the 3 adhesin-like genes ( YALI0C11165 , YALI0B18194 , and YALI0C23452 ), YALI0E22286 ( CWP1 -like gene), Yl PHR1 , and MHY1 were highly upregulated (≥5-fold) ( 9 ). MHY1 is a key regulator that promotes filamentation ( 15 , 16 ).…”

“…MHY1 is a key regulator that promotes filamentation ( 15 , 16 ). Yl PHR1 is required for filamentation at alkaline pH ( 9 ). Although cells deleted for the adhesin-like gene YALI0C23452 did not exhibit any defect in filamentation (data not shown), we observed that YALI0C23452 increased the cell length when overexpressed together with YALI0F19030 ( 9 ), another adhesin-like gene repressed by Fts2.…”

“…The CWP1 -like gene YALI0E22286 is upregulated by 564-fold by alkaline pH, which is very high. YALI0E22286 also increased the cell length when overexpressed together with YALI0D09185 ( 9 ), another adhesin-like gene repressed by Fts2. The repression of these major targets by Fts2 might be responsible for the repression of filamentation.…”

“…Under the induction of poor carbon source, nitrogen starvation, and alkaline pH, Y. lipolytica cells can switch from oval-shaped yeast form to filamentous forms including rod-like elongated cells, pseudohyphae, and hyphae ( 6 , 7 ). Previous studies have shown that filamentation in Y. lipolytica is positively regulated by evolutionarily conserved pathways such as the Ras/MAPK pathway and the Rim101 pH response pathway ( 8 , 9 ), which also function similarly in S. cerevisiae and C. albicans ( 10 – 13 ). Filamentation is also positively regulated by the transcription factors Mhy1 and Hoy1 ( 14 , 15 ).…”