RAS/Cyclic AMP and Transcription Factor Msn2 Regulate Mating and Mating-Type Switching in the Yeast Kluyveromyces lactis

In response to harsh environmental conditions, ascomycetes produce stress-resistant spores to promote survival. As sporulation requires a diploid DNA content, species with a haploid lifestyle, such asKluyveromyces lactis , first induce mating in response to stress. InK. lactis , mating and mating-type switching are induced by the DNA-binding protein Mts1. Mts1 expression is known to be upregulated by nutrient limitation, but the mechanism is unknown. We show that aras2 mutation results in a hyperswitching phenotype. In contrast, strains lacking the phosphodiesterase Pde2 had lower switching rates compared to that of the wild type (WT). As Ras2 promotes cyclic AMP (cAMP) production and Pde2 degrades cAMP, these data suggest that low cAMP levels induce switching. Because theMTS1 regulatory region contains several Msn2 binding sites and Msn2 is a transcription factor that is activated by low cAMP levels, we investigated if Msn2 regulatesMTS1 transcription. Consistently with this idea, anmsn2 mutant strain displayed lower switching rates than the WT strain. The transcription ofMTS1 is highly induced in theras2 mutant strain. In contrast, anmsn2 ras2 double mutant strain displays WT levels of theMTS1 transcript, showing that Msn2 is a critical inducer ofMTS1 transcription. Strains lacking Msn2 and Pde2 also exhibit mating defects that can be complemented by the ectopic expression of Mts1. Finally, we show thatMTS1 is subjected to negative autoregulation, presumably adding robustness to the mating and switching responses. We suggest a model in which Ras2/cAMP/Msn2 mediates the stress-induced mating and mating-type switching responses inK. lactis .