(A) Yeast and E. coli share hundreds of genes, 58 of which are essential in yeast and have clear 1:1 orthologs in either species. E. coli genes were cloned into a yeast expression vector under the control of a GPD promoter. 51 of these 58 E. coli genes provided informative assays for replaceability in yeast. Initial results from these complementation assays revealed that 25 of 51 (~49%) E. coli genes could functionally replace their orthologous yeast counterparts. (B) Complementation assays were performed in two different yeast strain backgrounds, as shown for representative assays. In the case of a yeast strain with a temperature-sensitive allele of the yeast gene Sc-cdc8, cells carrying the empty vector control grow at the permissive-temperature (25°C, yeast protein active) but not the restrictive-temperature (36°C, yeast protein inactive), unlike cells expressing the E. coli ortholog (Ec-tmK), indicating that the E. coli gene can functionally replace the yeast gene. In the case of yeast heterozygous diploid (Sc-ths1Δ/Sc-THS1) deletion strain, cells are sporulated and haploid progeny grown on selective medium (-Ura -Arg -His -Leu + Can) in the absence (yeast gene present) or presence of G418 (200 μg/ml) (yeast gene absent). Cells expressing the E. coli ortholog (Ec-thrS) grow on G418-containing medium, unlike cells carrying the empty vector control, indicating successful complementation. (C) Haploid yeast gene deletion strains carrying plasmids expressing functionally replacing E. coli genes (red solid-lines) generally exhibit comparable growth rates to the wild type parental yeast strain BY4741 (black dotted-lines). The empty vector control (grey solid-line) showed no such growth rescue in the presence of G418. Mean and standard deviation plotted with N = 3.