Molecular insights into FucR transcription factor to control the metabolism of L -fucose in Bifidobacterium longum subsp. infantis.

Bifidobacterium longum subsp. infantis commonly colonizes the human gut and is capable of metabolizing L - fucose, which is abundant in the gut. Multiple studies have focused on the mechanisms of L -fucose utilization by B. longum subsp. infantis, but the regulatory pathways governing the expression of these catabolic processes are still unclear. In this study, we have conducted a structural and functional analysis of L - fucose metabolism transcription factor FucR derived from B. longum subsp. infantis Bi-26. Our results indicated that FucR is a L - fucose-sensitive repressor with more α-helices, fewer β-sheets, and β-turns. Transcriptional analysis revealed that FucR displays weak negative self-regulation, which is counteracted in the presence of L -fucose. Isothermal titration calorimetry indicated that FucR has a 2:1 stoichiometry with L - fucose. The key amino acid residues for FucR binding L - fucose are Asp280 and Arg331, with mutation of Asp280 to Ala resulting in a decrease in the affinity between FucR and L - fucose with the Kd value from 2.58 to 11.68 μM, and mutation of Arg331 to Ala abolishes the binding ability of FucR towards L - fucose. FucR specifically recognized and bound to a 20-bp incomplete palindrome sequence (5'-ACCCCAATTACGAAAATTTTT-3'), and the affinity of the L - fucose-loaded FucR for the DNA fragment was lower than apo-FucR. The results provided new insights into the regulating L -fucose metabolism by B. longum subsp. infantis.