Modeling in yeast how rDNA introns slow growth and increase desiccation tolerance in lichens

Abstract

We define a molecular connection between ribosome biogenesis and desiccation tolerance in lichens, widespread symbioses between specialized fungi (mycobionts) and unicellular phototrophs. Our experiments test whether the introns present in the nuclear ribosomal DNA of lichen mycobionts contribute to their anhydrobiosis. Self-splicing introns are found in the rDNA of several eukaryotic microorganisms, but most introns populating lichen rDNA are unable to self-splice, being either degenerate group I introns lacking the sequences needed for catalysis, or spliceosomal introns ectopically present in rDNA. Although all introns are eventually removed from rRNA by the splicing machinery of the mycobiont, Northern analysis of its RNA indicates that they are not removed quickly during rRNA transcription but are still present in early post-transcriptional processing and ribosome assembly stages, suggesting that delayed splicing interferes with ribosome assembly. To study the phenotypic repercussions of lichen introns in a model system, we used CRISPR to introduce a spliceosomal intron from the rDNA of the lichen fungus Cladonia grayi into all nuclear rDNA copies of the yeast Saccharomyces cerevisiae, which lacks rDNA introns. Three intron-bearing yeast mutants were constructed with the intron inserted either in the 18S rRNA genes, the 25S rRNA genes, or in both. The mutants removed the introns correctly but had half the rDNA genes of the wildtype strain, grew 4.4 to 6 times slower, and were 40 to 1700 times more desiccation tolerant depending on intron position and number. Intracellular trehalose, a disaccharide implicated in desiccation tolerance, was detected, but at low concentration. Overall, our data suggest that the constitutive interference of the intron splicing machinery with ribosome assembly and the consequent lowering of the cytoplasmic concentration of ribosomes and proteins are the primary causes of slow growth and increased desiccation tolerance in the yeast mutants. The relevance of these findings for slow growth and desiccation tolerance in lichens is discussed.