12 DNA variants that alter gene expression contribute to variation in many phenotypic traits. In 13 particular, trans-acting variants, which are often located on different chromosomes from the genes 14 they affect, are an important source of heritable gene expression variation. However, our 15 knowledge about the identity and mechanism of causal trans-acting variants remains limited. Here, 16 we developed a fine-mapping strategy called CRISPR-Swap and dissected three expression 17 quantitative trait locus (eQTL) hotspots known to alter the expression of numerous genes in trans 18 in the yeast Saccharomyces cerevisiae. Causal variants were identified by engineering 19 recombinant alleles and quantifying the effects of these alleles on the expression of a green 20 fluorescent protein-tagged gene affected by the given locus in trans. We validated the effect of 21 each variant on the expression of multiple genes by RNA-sequencing. The three variants were 22 strikingly different in their molecular mechanism, the type of genes they reside in, and their 23 distribution in natural populations. While a missense leucine-to-serine variant at position 63 in the 24 transcription factor Oaf1 (L63S) was almost exclusively present in the reference laboratory strain, 25 the two other variants were frequent among S. cerevisiae isolates. A causal missense variant in the 26 glucose receptor Rgt2 (V539I) occurred at a poorly conserved amino acid residue and its effect 27 was strongly dependent on the concentration of glucose in the culture medium. A noncoding 28 variant in the conserved fatty acid regulated (FAR) element of the OLE1 promoter influenced the 2 29 expression of the fatty acid desaturase Ole1 in cis and, by modulating the level of this essential 30 enzyme, other genes in trans. The OAF1 and OLE1 variants showed a non-additive genetic 31 interaction, and affected cellular lipid metabolism. These results revealed remarkable diversity in 32 the molecular basis of trans-regulatory variation, highlighting the challenges in predicting which 33 natural genetic variants affect gene expression.
34Author summary 35 Differences in the DNA sequence of individual genomes contribute to differences in many traits, 36 such as appearance, physiology, and the risk for common diseases. An important group of these 37 DNA variants influences how individual genes across the genome are turned on or off. In this 38 paper, we describe a strategy for identifying such "trans-acting" variants in different strains of 39 baker's yeast. We used this strategy to reveal three single DNA base changes that each influences 40 the expression of dozens of genes. These three DNA variants were very different from each other. 41 Two of them changed the protein sequence, one in a transcription factor and the other in a sugar 42 sensor. The third changed the expression of an enzyme, a change that in turn caused other genes 43 to alter their expression. One variant existed in only a few yeast isolates, while the other two 44 existed in many isolates collected fro...