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

Abstract

We connect ribosome biogenesis to desiccation tolerance in lichens, widespread symbioses between specialized fungi (mycobionts) and unicellular phototrophs. We 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 catalytically-impaired group I introns, or spliceosomal introns ectopically present in rDNA. Although the mycobiont’s splicing machinery removes all introns from rRNA, Northern analysis indicates delayed post-transcriptional removal during rRNA processing, suggesting interference with ribosome assembly. To study the effects of lichen introns in a model system, we used CRISPR to introduce a spliceosomal rDNA intron from the lichen fungus Cladonia grayi into all nuclear rDNA copies of 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, 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 at low concentration. Our data suggest that the interference of the splicing machinery with ribosome assembly leads to fewer ribosomes and proteins and to 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.

Regulation of symbiotic interactions and primitive lichen differentiation by UMP1 MAP kinase in Umbilicaria muhlenbergii

“…To better understand mycobiont-photobiont symbiosis, it is important to characterize molecular mechanisms governing their interactions, establishment of initial symbiotic complexes, and development of primitive and mature lichen thalli. Unfortunately, most lichen-forming fungi are not amenable to molecular genetic studies, and artificial synthesis of lichens with isolated mycobionts and photobionts is difficult due to their slow growth rate 6 , 7 .…”

Regulation of symbiotic interactions and primitive lichen differentiation by UMP1 MAP kinase in Umbilicaria muhlenbergii

“…To better understand mycobiont-photobiont symbiosis, it is important to characterize molecular mechanisms governing their interactions, establishment of initial symbiotic complexes, and development of primitive and mature lichen thalli. Unfortunately, most lichenforming fungi are not amenable to molecular genetic studies, and artificial synthesis of lichens with isolated mycobionts and photobionts is difficult due to their slow growth rate 6,7 .…”