Flavin transferase that catalyzes the transfer of the FMN moiety of FAD and its covalent binding to the hydroxyl group of a threonine residue in a target flavoprotein. Is responsible for the modification of the fumarate reductase KPK_2907. FAD + L-threonyl-[protein] = AMP + FMN-L-threonyl-[protein] + H(+) Inactivation of this gene results in a complete loss of the fumarate reductase activity displayed by KPK_2907, which contains a covalently bound FMN, but does not affect the Na(+)-NQR activity. Belongs to the ApbE family.