TOR Pathway Allows Yeast to Optimize Their Genome in Response to Environmental cChanges
It is generally accepted that changes to a cell’s genome are driven by random mutation. Changes that confer a growth advantage become established in a population through the process of natural selection. A study published in PNAS suggests that cells can play a more active role in the evolution of their genomes — in response to caloric excess the TOR pathway in budding yeast initiates an expansion of the number of ribosomal genes.
Jack et al used the budding yeast Saccharomyces cerevisiae to follow up reports that some organisms can expand their ribosomal gene copy number in response to changes in environmental conditions. This phenomenon does not fit conventional ideas about evolution because in yeast an increased number of ribosomal genes does not confer a measurable growth advantage under laboratory conditions. This suggested that a mechanism exists to assess and alter rDNA copy number in response to environmental factors.
Jack et al focused on the TOR pathway because it is known to repress ribosomal gene expansion during caloric restriction. Their results show that rDNA amplification is mediated by TOR in response to caloric excess and occurs through at least two different recombination pathways – the homologous recombination-dependent BIR pathway and a non-homologous recombination-dependent pathway that is inhibited by the histone deacetylases Hst3 and Hst4. The authors suggest that these results are the first demonstration of a signaling pathway that directly regulates copy number and hypothesize that other regions of the genome may also be modified in response to environmental conditions. Because the TOR pathway is conserved in higher eukaryotes these findings suggest that the ability to optimize genomes in a directed fashion may be a widespread phenomenon.