Two broadly conserved families of polyprenyl-phosphate transporters.

Peptidoglycan and virtually all surface glycopolymers in bacteria are built in the cytoplasm on the lipid carrier undecaprenyl-phosphate (UndP) 1-4 . These UndP-linked precursors are transported across the membrane and polymerized or directly transferred to surface polymers, lipids, or proteins. UndP is then flipped to regenerate the pool of cytoplasmic-facing UndP. The identity of the flippase that catalyzes transport has eluded identification for decades. Here, using the antibiotic amphomycin that targets UndP 5-7 , we discovered two broadly conserved protein families that affect UndP recycling. One (UptA) is a member of the DedA superfamily 8 ; the other (PopT) contains the domain DUF368. Genetic, cytological, and syntenic analyses argue that these proteins are the missing UndP transporters. Importantly, homologs from gram-positive and gram-negative bacteria promote UndP transport in Bacillus subtilis, indicating that recycling activity is broadly conserved among family members. Inhibitors of these flippases could potentiate the current arsenal of cell envelope-targeting antibiotics.