Checks and balances of the RTG pathway under arginine deprivation and canavanine exposure in Saccharomyces cerevisiae

Decades of research in Saccharomyces cerevisiae underlie the current dogma of mitochondrial retrograde (RTG) signaling: Rtg2-dependent translocation of the heterodimer Rtg1/Rtg3 from the cytoplasm to the nucleus induces the transcription of RTG-target genes under glutamate starvation or loss of respiration. We previously found that RTG mutants show severe growth inhibition from arginine deprivation and are highly sensitive to canavanine when grown on glucose. Here, we show that on solid media, RTG mutants are also sensitive to thialysine, a toxic lysine analog, although lysine deprivation causes a milder growth defect. Growth on an alternative carbon source restores RTG mutants ability to grow without arginine or lysine and improves their tolerance of toxic analogs; deletion of MIG1 affords a similar rescue on glucose and improves canavanine tolerance, except for in rtg2{Delta}. It is well known that the target of rapamycin (TOR) signaling pathway inhibits the RTG pathway. Batch growth experiments with or without TOR inhibition reveal phenotypic and regulatory differences between RTG mutants. rtg1{Delta} can sustain simultaneous canavanine exposure and TOR inhibition via rapamycin, but rtg2{Delta} and rtg3{Delta} cannot. Surprisingly, our data show that under fermentative lifestyle and arginine deprivation, both RTG signaling and TOR activity are required. This expands the universe of TOR and RTG signaling, suggesting bilateral communication rather than unidirectional RTG regulation by TOR. To the best of our knowledge, this work shows for the first time that Rtg3 activity can be separate from its role as a heterodimer with Rtg1. This work also strongly suggests a specific role for Rtg2 in canavanine tolerance.