A Rac homolog is required for induction of hyphal growth in the dimorphic yeast Yarrowia lipolytica

Dimorphism in fungi is believed to constitute a mechanism of response to ad verse conditions and represents an important attribute for the development of virulence by a number of pathogenic fungal species. We have isolated YlR -AC1, a gene encoding a 192-amino-acid protein that is essential for hyphal growth in the dimorphic yeast Yarrowia lipolytica and which represents the first Rac homolog described far fungi. YlRAC1 is not an essential gene, an d its deletion does not affect the ability to mate or impair actin polariza tion in Y. lipolytica. However, strains lacking functional YlRAC1 show alte rations in cell morphology suggesting that the function of YlRAC1 may be re lated to some aspect of the polarization of cell growth. Northern blot anal ysis show ed that transcription of YlRAC1 increases steadily during the yea st-to-hypha transition, while Southern blot analysis of genomic DNA suggest ed the presence of several RAC family members in Y. lipolytica. Interesting ly, strains lacking functional YlRAC1 are still able to grow as the pseudoh yphal form and to invade agar, thus pointing to a function for YlRAC1 downs tream of MHY1, a previously isolated gene encoding a C2W2 type zinc finger protein with the ability to bind putative stress response elements and whos e activity is essential for both hyphal and pseudohyphal growth in Y. lipol ytica.