The Biosynthesis of Flavin Cofactors in Listeria monocytogenes

Listeria monocytogenes is riboflavin auxotrophic, but it has two genes envisaged to transform riboflavin into FMN and FAD after its uptaked by specialized transporters. One encodes a bifunctional type I FAD synthase (FADS, herein LmFADS-1), while the other produces a protein similar to type I at the FMN:ATP adenylyltransferase (FMNAT) site but with a shorter C-terminal that lacks any riboflavin kinase (RFK) motif. This second protein is rare among bacteria and has been named FADS type II (LmFADS-2). Here we present a biochemical and biophysical study of LmFADS-1 and LmFADS-2 by integrating kinetic and thermodynamic data together with sequence and structural prediction methods to evaluate their occurrence in Listeria, as well as their function and molecular properties. Despite LmFADS-1 similarities to other type I FADSs, (i) its RFK activity has not riboflavin substrate inhibition and occurs under reducing and oxidizing conditions, (ii) its FMNAT activity requires strong reducing environment, and (iii) binding of reaction products, but not substrates, favors binding of the second ligand. LmFADS-2 produces FAD under oxidizing and reducing environments, but its C-terminus module function remains unknown. Listeria species conserve both FADSs, being sequence identity high within L. monocytogenes strains. Our data exemplify alternative strategies for FMN and FAD biosynthesis and homeostasis, envisaging that in Listeria two FADSs might be required to fulfill the supply of flavin cofactors under niches that can go from saprophytism to virulence. As FADSs are attractive antimicrobial targets, understanding of FADSs traits in different species is essential to help in the discovery of specific antimicrobials.