Flavodoxins (Flds) are oxidoreductases that distribute electrons to different metabolic pathways through interactions with an array of partner proteins. The aim of my thesis is to understand Fld evolution, establish whether Flds are encoded within the same genomes as Fd-dependent sulfite reductases (SIRs), and demonstrate that a cellular assay can monitor Fld electron transfer (ET) to SIRs. Using bioinformatics, I identify numerous microbes whose genomes encode both Fld and SIR genes. Additionally, I show that Flds can support ET to SIR using a synthetic pathway where protein-mediated ET is monitored using the growth of an Escherichia coli auxotroph that depends upon Fld transferring electrons from a Fd:NADP+ reductase to SIR. My results represent the first evidence that Flds support ET to assimilatory SIRs. Additionally, they show how a synthetic ET pathway in cells can be leveraged to rapidly compare the ET efficiencies of different Flds.