Bifunctional enzyme that catalyzes the oxidative decarboxylation of UDP-glucuronic acid (UDP-GlcUA) to UDP-4-keto-arabinose (UDP-Ara4O) and the addition of a formyl group to UDP-4-amino-4-deoxy-L-arabinose (UDP-L-Ara4N) to form UDP-L-4-formamido-arabinose (UDP-L-Ara4FN). The modified arabinose is attached to lipid A and is required for resistance to polymyxin and cationic antimicrobial peptides. NAD(+) + UDP-alpha-D-glucuronate = CO2 + NADH + UDP-beta-L-threo-pentopyranos-4-ulose (6S)-10-formyltetrahydrofolate + UDP-4-amino-4-deoxy-beta-L-arabinose = (6S)-5,6,7,8-tetrahydrofolate + H(+) + UDP-4-deoxy-4-formamido-beta-L-arabinose Nucleotide-sugar biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose from UDP-alpha-D-glucuronate: step 1/3. Nucleotide-sugar biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose biosynthesis; UDP-4-deoxy-4-formamido-beta-L-arabinose from UDP-alpha-D-glucuronate: step 3/3. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homohexamer, formed by a dimer of trimers. Induced by BasR. In the N-terminal section; belongs to the Fmt family. UDP-L-Ara4N formyltransferase subfamily. In the C-terminal section; belongs to the NAD(P)-dependent epimerase/dehydratase family. UDP-glucuronic acid decarboxylase subfamily.