Genomic Instability and Adaptive Evolution Induced by RFA Insufficiency in Saccharomyces cerevisiae

This study systematically investigated the genomic alterations in <i>Saccharomyces cerevisiae</i> driven by Replication Factor A (RFA) dosage insufficiency using a promoter-replacement strategy combined with mutation accumulation and whole-genome sequencing. Our findings reveal that transcriptional suppression of <i>RFA2</i> or <i>RFA3</i> leads to severe growth inhibition. RFA deficiency induces a distinct mutational spectrum characterized by a high frequency of monosomy and terminal deletions, indicative of severe replication stress. Furthermore, loss of heterozygosity is significantly enriched at centromeres and high-GC regions, underscoring the role of RFA in stabilizing intrinsic genomic barriers. Utilizing an APOBEC3B-induced mutagenesis assay, we demonstrate that RFA insufficiency leads to the extensive accumulation of exposed ssDNA with a distinct bias towards the lagging strand template. Notably, we observed that cells spontaneously inactivate Mismatch Repair (MMR) genes, such as <i>MSH2</i> and <i>PMS1</i>, to survive RFA-induced stress. This hypermutant phenotype grants a certain degree of growth recovery on Low Galactose (LG) medium. Overall, these findings demonstrate that RFA dosage is a key determinant of genomic integrity and elucidate how repair pathway modulation drives adaptive evolution under replication stress.