Laboratory evolution identifies elongated flavodoxins that support electron transfer to sulfite reductases
Flavodoxins (Flds) mediate the flux of electrons between oxidoreductases in diverse metabolic pathways. While dozens of Fld-partner oxidoreductases have been discovered, these only represent a subset of the oxidoreductases that couple with ferredoxin (Fd) protein electron carriers. To investigate whether Flds can support electron transfer to a sulfite reductase (SIR) that evolved to couple with a Fd, we evaluated the ability of Flds to transfer electrons from a Fd-NADP reductase (FNR) to a Fd-dependent SIR using growth complementation of a microbe with a sulfur metabolism defect. We show that Flds from cyanobacteria complement the growth of this microbe when coexpressed with an FNR and an SIR that evolved to couple with a plant Fd. To better understand the interaction of Fld with these partner oxidoreductases, we evaluated the effect of peptide insertion on Fld-mediated electron transfer. We observe a high insertion sensitivity within regions predicted to be proximal to the cofactor and partner binding sites and a high insertion tolerance within the loop that is used to differentiate short- and long-chain flavodoxins. These results represent the first evidence that Flds can support electron transfer to assimilatory SIRs, and they suggest that the pattern of peptide-insertion tolerance is influenced by interactions with oxidoreductase partners in electron transfer pathways.