Epistasis, the modification of the fitness effects of a mutation by other mutations, has important consequences for the trajectory of adaptive evolution and for the shape of the “adaptive landscape”. Nonetheless, the extent of epistasis, and the shape of the distribution of epistatic effects (DoEE), are poorly characterized. We developed a high-throughput method for describing the DoEE. A set of 4123 E. coli ORFs (the ASKA collection) was transformed into a strain bearing a common antibiotic resistance mutation, and into a wild-type strain. Upon induction of expression, the fitness of each
ORF was determined using a sequence-based assay, providing a rapid measure of background-dependent fitness effects for over 4000 constructs. 216 epistatic interactions were detected. Synthetic dosage lethal/sick interactions from the over-expression of yiaN and nfi were validated using 24 hour growth curves, and may indicate that these genes are potential targets for drug design.