Molecular diversity of the genetic loci responsible for lipopolysaccharide core oligosaccharide assembly within the genus Salmonella

Abstract

The waa locus on the chromosome of Salmonella enterica encodes enzymes involved in the assembly of the core oligosaccharide region of the lipopolysaccharide (LPS) molecule. To date, there are two known core structures in Salmonella, represented by serovars Typhimurium (subspecies I) and Arizonae (subspecies IIIA). The waa locus for serovar Typhimurium has been characterized. Here, the corresponding locus from serovar Arizonae is described, and the molecular basis for the distinctive structures is established. Eleven of the 13 open reading frames (ORFs) are shared by the two loci and encode conserved proteins of known function. Two polymorphic regions distinguish the waa loci. One involves the waaK gene, the product of which adds a terminal alpha-1,2-linked N-acetylglucosamine residue that characterizes the serovar Typhimurium core oligosaccharide. There is an extensive internal deletion within waaK of serovar Arizonae. The serovar Arizonae locus contains a novel ORF (waaH) between the waaB and waaP genes. Structural analyses and in vitro glycosyltransferase assays identified WaaH as the UDP-glucose:(glucosyl) LPS alpha-1,2-glucosyltransferase responsible for the addition of the characteristic terminal glucose residue found in serovar Arizonae. Isolates comprising the Salmonella Reference Collections, SARC (representing the eight subspecies of S. enterica) and SARB (representing subspecies I), were examined to assess the distribution of the waa locus polymorphic regions in natural populations. These comparative studies identified additional waa locus polymorphisms, shedding light on the genetic basis for diversity in the LPS core oligosaccharides of Salmonella isolates and identifying potential sources of further novel LPS structures.