The foodborne bacterial pathogen, Campylobacter jejuni, possesses an N-linked protein glycosylation (pgl) pathway involved in adding conserved heptasaccharides to asparagine-containing motifs of >60 proteins, and releasing the same glycan into its periplasm as free oligosaccharides. In this study, comparative genomics of all 30 fully sequenced Campylobacter taxa revealed conserved pgl gene clusters in all but one species. Structural, phylogenetic and immunological studies showed that the N-glycosylation systems can be divided into two major groups. Group I includes all thermotolerant taxa, capable of growth at the higher body temperatures of birds, and produce the C. jejuni-like glycans. Within group I, the niche-adapted C. lari subgroup contain the smallest genomes among the epsilonproteobacteria, and are unable to glucosylate their pgl pathway glycans potentially reminiscent of the glucosyltransferase regression observed in the O-glycosylation system of Neisseria species. The nonthermotolerant Campylobacters, which inhabit a variety of hosts and niches, comprise group II and produce an unexpected diversity of N-glycan structures varying in length and composition. This includes the human gut commensal, C. hominis, which produces at least four different N-glycan structures, akin to the surface carbohydrate diversity observed in the well-studied commensal, Bacteroides. Both group I and II glycans are immunogenic and cell surface exposed, making these structures attractive targets for vaccine design and diagnostics. Molecular & Cellular Proteomics 11: 10.1074/mcp.M112.021519, 1203-1219, 2012.In eukaryotes, glycosylated proteins are ubiquitous components of extracellular matrices and cellular surfaces. Their oligosaccharide moieties are implicated in a wide variety of essential cell-cell and cell-matrix processes ranging from immune recognition to cancer development. The first general protein glycosylation (pgl) 1 pathway was discovered in the epsilonproteobacterium Campylobacter jejuni (1). The organism transfers a conserved heptasaccharide en bloc to asparagine residues within the sequon D/E-X 1 -N-X 2 -S/T (X 1 , X 2 P) of Ͼ60 glycoproteins (2-4). Furthermore, the pathway can be functionally transferred into Escherichia coli, and the oligosaccharyltransferase (OTase), PglB, is capable of adding foreign sugars to acceptor proteins (5-7). C. jejuni PglB also possesses hydrolase activity, influenced by the cellular growth phase and osmotic environment, releasing free oligosaccharides (fOS) into the periplasmic space in a 10:1 ratio relative to the amount of heptasaccharide N-linked to protein (8, 9).The C. jejuni N-linked heptasaccharide is conserved in structure in both C. jejuni and C. coli, the two most commonly isolated pathogenic Campylobacter species and major causes of human enteritis worldwide (10, 11). All campylobacters, but one, possess conserved pgl genes required for Nlinked protein glycosylation ((12) and this study). This posttranslational modification in C. jejuni influences DNA uptake, chicken and ...