Sialic Acid ( N -Acetyl Neuraminic Acid) Utilization by Bacteroides fragilis Requires a Novel N -Acetyl Mannosamine Epimerase

We characterized thenanLET operon inBacteroides fragilis , whose products are required for the utilization of the sialic acidN- acetyl neuraminic acid (NANA) as a carbon and energy source. The first gene of the operon isnanL , which codes for an aldolase that cleaves NANA intoN -acetyl mannosamine (manNAc) and pyruvate. The next gene,nanE , codes for a manNAc/N -acetylglucosamine (NAG) epimerase, which, intriguingly, possesses more similarity to eukaryotic renin binding proteins than to other bacterial NanE epimerase proteins. Unphosphorylated manNAc is the substrate of NanE, while ATP is a cofactor in the epimerase reaction. The third gene of the operon isnanT , which shows similarity to the major transporter facilitator superfamily and is most likely to be a NANA transporter. Deletion of any of these genes eliminates the ability ofB. fragilis to grow on NANA. AlthoughB. fragilis does not normally grow with manNAc as the sole carbon source, we isolated aB. fragilis mutant strain that can grow on this substrate, likely due to a mutation in a NAG transporter; both manNAc transport and NAG transport are affected in this strain. Deletion of thenanE epimerase gene or therokA hexokinase gene, whose product phosphorylates NAG, in the manNAc-enabled strain abolishes growth on manNAc. Thus,B. fragilis possesses a new pathway of NANA utilization, which we show is also found in otherBacteroides species.