Bacteroidota polysaccharide utilization system for branched dextran exopolysaccharides from lactic acid bacteria

Area of Science:

• Microbiology
• Biochemistry
• Structural Biology

Background:

• Dextran, an α-(1→6)-glucan, is produced by lactic acid bacteria, often with α-(1→2)-, α-(1→3)-, and α-(1→4)-linkages.
• Bacterial degradation mechanisms for branched dextrans remain largely uncharacterized.
• The dextran utilization locus (FjDexUL) in Flavobacterium johnsoniae was previously identified.

Purpose of the Study:

• To functionally analyze the FjDexUL proteins involved in branched dextran degradation.
• To elucidate the mechanism by which *Flavobacterium johnsoniae* utilizes branched dextrans.
• To understand bacterial nutrient requirements and symbiotic relationships.

Main Methods:

• Gene expression analysis of FjDexUL under different carbon sources.
• Biochemical characterization of FjDexUL glycoside hydrolases.
• Crystal structure determination of FjGH66 and FjGH65A.
• Characterization of cell surface sugar-binding proteins FjDusD and FjDusE.

Main Results:

• FjDexUL proteins were shown to recognize and degrade α-(1→2)- and α-(1→3)-branched dextrans.
• FjDexUL gene expression was upregulated in the presence of S-32 α-glucan.
• Glycoside hydrolases within FjDexUL acted synergistically.
• Structural analysis revealed sugar-binding subsites accommodating branched linkages.
• FjDusD and FjDusE exhibited specific affinities for isomaltooligosaccharides and dextrans, respectively.

Conclusions:

• The FjDexUL locus plays a crucial role in the degradation of α-(1→2)- and α-(1→3)-branched dextrans.
• These findings enhance understanding of bacterial nutrient acquisition and inter-bacterial molecular interactions.