Vitamin B12-peptide nucleic acids use the BtuB receptor to pass through the Escherichia coli outer membrane

Abstract

Short modified oligonucleotides that bind in a sequence-specific way to messenger RNA essential for bacterial growth could be useful to fight bacterial infections. One such promising oligonucleotide is peptide nucleic acid (PNA), a synthetic DNA analog with a peptide-like backbone. However, the limitation precluding the use of oligonucleotides, including PNA, is that bacteria do not import them from the environment. We have shown that vitamin B12, which most bacteria need to take up for growth, delivers PNAs to Escherichia coli cells when covalently linked with PNAs. Vitamin B12 enters E. coli via a TonB-dependent transport system and is recognized by the outer-membrane vitamin B12-specific BtuB receptor. We engineered the E. coli ΔbtuB mutant and found that transport of the vitamin B12-PNA conjugate requires BtuB. Thus, the conjugate follows the same route through the outer membrane as taken by free vitamin B12. From enhanced sampling all-atom molecular dynamics simulations, we determined the mechanism of conjugate permeation through BtuB. BtuB is a β-barrel occluded by its luminal domain. The potential of mean force shows that conjugate passage is unidirectional and its movement into the BtuB β-barrel is energetically favorable upon luminal domain unfolding. Inside BtuB, PNA extends making its permeation mechanically feasible. BtuB extracellular loops are actively involved in transport through an induced-fit mechanism. We prove that the vitamin B12 transport system can be hijacked to enable PNA delivery to E. coli cells.