Involved in the lipopolysaccharide (LPS) O-antigen biosynthesis (PubMed:11081580). Catalyzes the transfer of a formyl group to GDP-perosamine, leading to the formation of GDP-N-formylperosamine (PubMed:11081580, PubMed:28636341). In vitro, can also function on GDP-3-deoxyperosamine, thereby providing an enzymatic method for producing a novel trideoxysugar not found in nature (PubMed:28636341). (6S)-10-formyltetrahydrofolate + GDP-alpha-D-perosamine = (6S)-5,6,7,8-tetrahydrofolate + GDP-N-formyl-alpha-D-perosamine + H(+) Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homodimer. Deletion mutant is completely O-antigen-negative. Belongs to the Fmt family.