The nucleotide sequence of a 23S rRNA gene of Campylobacter coli VC167 was determined. The primary sequence of the C. coli 23S rRNA was deduced, and a secondary-structure model was constructed. Comparison with Escherichia coli 23S rRNA showed a major difference in the C. coli rRNA at approximately position 1170 (E. coli numbering) in the form of an extra sequence block approximately 147 bp long. PCR analysis of 31 other strains of C. coli and C. jejuni showed that 69%o carried a transcribed spacer of either ca. 147 or ca. 37 bp.Comparison of all sequenced Campylobacter transcribed spacers showed that the Campylobacter inserts were related in sequence and percent G+C content. All Campylobacter strains carrying transcribed spacers in their 23S rRNA genes produced fragmented 23S rRNAs. Other strains which produced unfragmented 23S rRNAs did not appear to carry transcribed spacers at this position in their 23S rRNA genes. At the 1850 region (E. coli numbering), Campylobacter 23S rRNA displayed a base pairing signature most like that of the beta and gamma subdivisions of the class Proteobacteria, but in the 270 region, Campylobacter 23S rRNA displayed a helix signature which distinguished it from the alpha, beta, and gamma subdivisions. Phylogenetic analysis comparing C. coli VC167 23S rRNA and a C.jejuni TGH9011 (ATCC 43431) 23S rRNA with 53 other completely sequenced (eu)bacterial 23S rRNAs showed that the two campylobacters form a sister group to the alpha, beta, and gamma proteobacterial 23S rRNAs, a positioning consistent with the idea that the genus Campylobacter belongs to the epsilon subdivision of the class Proteobacteria.The members of the Campylobacter group of organisms are important pathogens in human and veterinary medicine. This group includes Campylobacter, Arcobacter, Helicobacter, Wolinella, and "Flexispira" spp. and has been argued to constitute a separate bacterial lineage within the class Proteobacteria (62, 63). The most recently published phylogenetic tree based on small-subunit rRNA sequences has the group positioned in the delta and epsilon subdivisions of the Proteobacteria (purple bacteria) (39). There are 11 Campylobacter species (9, 56), and the roles of several of these species in diseases of humans and animals have been well described (43). Typically, these small, nutritionally fastidious, microaerophilic, gram-negative spiral bacteria colonize moist body surfaces, including the human gingival cavity, the small intestinal mucosa, and the vaginal mucosa of bovines and this colonization leads to disease. The two most important human pathogens in the genus are Campylobacterjejuni and C. coli, which cause enteritis.In keeping with their small size and limited nutritional properties, the campylobacters have a small genome (23,37,57). At 1.7 to 1.8 Mb, the C. jejuni and C. coli genomes are only 36% of the size of the Escherichia coli chromosome (53) (23,24,27,37,57). Unlike the characteristic continuous 16S-23S (eu)bacterial rRNA operon structure seen in E. coli (35), several studies have sug...