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 (By similarity). Is likely involved in the modification of RnfG and RnfD. Required for nitrogen fixation (PubMed:8264535). FAD + L-threonyl-[protein] = AMP + FMN-L-threonyl-[protein] + H(+) In the N-terminal section; belongs to the RseC family. In the C-terminal section; belongs to the ApbE family.