YlAaf1-YlAaf2, a bipartite SANT domain-containing complex of transcriptional activator, promotes filamentous growth in the dimorphic yeast <i>Yarrowia lipolytica</i>

The yeast-to-filament transition is important for dimorphic fungi to adapt to different environmental conditions. Transcriptional activators that activate downstream target genes in response to environmental stimulations are particularly important for the control of this morphological transition. However, only a limited number of key activators of filamentation have been identified. Here, we report YlAaf1 and YlAaf2, a novel pair of SANT domain-containing transcription factors that promote filamentation in the dimorphic yeast <i>Yarrowia lipolytica</i>. We show that both deletion and co-overexpression of Yl<i>AAF1</i> and Yl<i>AAF2</i> affect filamentation substantially. We also show that YlAaf1 and YlAaf2 interact with each other physically, and the bipartite complex has transcriptional activation activity. In addition, we show that YlAaf1 and YlAaf2 respond to alkaline pH and are upregulated by YlRim101 and Mhy1, two key transcription factors important for alkaline-induced filamentation. Lastly, we show that YlAaf1-YlAaf2 and Mhy1 are direct regulatory targets of each other.IMPORTANCETranscription factors play critical roles in the control of the yeast-to-filament transition in dimorphic fungi. However, many of them contain zinc finger or zinc cluster DNA-binding domains, but not SANT domain, the latter of which is initially identified in nuclear receptor co-repressors and the subunits of many chromatin-remodeling complexes. In this paper, we report two novel SANT domain-containing transcription factors, YlAaf1 and YlAaf2, that promote filamentation in <i>Yarrowia lipolytica</i>. YlAaf1 and YlAaf2 are closely related to <i>Candida albicans</i> Aaf1, which also appears to promote filamentation. YlAaf1 and YlAaf2 are unique in that they are not simply redundant in function but instead form a bipartite transcriptional activator. One subunit, YlAaf2, is highly responsive to environmental stimulation such as alkaline pH, a strong inducer of the yeast-to-filament transition. We identified a crosstalk between YlAaf1-YlAaf2 and Mhy1, the master regulator of filamentation. Our results provide new insights into the regulatory networks that control the yeast-to-filament transition in <i>Y. lipolytica</i>.