Extracellular electron transfer chain, (EET) possibly consisting of an 8 protein system, all constituents of which are encoded within a single gene cluster (Light et al. 2018). Some of these proteins may serve as auxiliary proteins rather than direct electron transfer proteins. One such protein is AbpE (i.e., Q8XQC0; DUF2271) which posttranslationally flavinylates EET proteins (Méheust et al. 2021). Flavin transferases catalyze the transfer of the FMN moiety of FAD to the hydroxyl group of a threonine residue in a target flavoprotein. See family description for more details.