Wendy Wu ’22
Each organism has genes adapted for survival to their environment. But over many generations, the environment may change. In the absence of selective pressures, the expression of certain genes can become too costly to maintain. Through evolution, these genes lose function no matter how beneficial they can be. The ability to regain that function when pressure is once again applied is not well understood. Mirna Kheir Gouda, a biomedical engineering student at Stony Brook University, aims to study this process, known as evolutionary reversibility, and to induce it in haploid Saccharomyces cerevisiae cells.
Kheir Gouda’s team focused on a positive-feedback (PF) gene circuit that was originally integrated into the S. cerevisiae YPH500 genome. This circuit coded for the protein, reverse tetracycline Transcriptional Activator (rtTA), which is important to antibiotic resistance. The team took seven rtTA mutants and tested the extent of the mutations by hyperinducing each mutant with excess doxycycline. Two mutants were found to be quasifunctional (low gene expression), one was dysfunctional (improper gene expression), while the remaining four were nonfunctional (no gene expression). Researchers then evolved the mutants under the condition D2Z2, 2 µg/mL of the inducer, doxycycline, and 2 mg/mL of the antibiotic, Zeocin. Growth observations were noted over the course of 14 days.
Evolution only directly restored the PF circuit in the dysfunctional mutant. In the other mutants, mutations that improved drug resistance occurred outside the circuit; this indicates that, if available, evolution could choose an alternate path to survival. Further research could determine if restriction of these alternate paths would force evolution to restore the original trait. Interestingly, certain extracircuit mutations elevated rtTA expression, indirectly stimulating the PF gene circuit. Additionally, the restored PF circuit in the dysfunctional mutant had evolved to be more robust and less costly. These results hold strong implications for synthetic biology as well as future drug resistance studies.
 M. Kheir Gouda, et al., Evolutionary regain of lost gene circuit function. Proceedings of the National Academy of Sciences of the United States of America 116, 25162-25171 (2019). doi:10.1073/pnas.1912257116.
 Image retrieved from: https://www.sciencesource.com/archive/Saccharomyces-cerevisiae-yeast-SS286690.html