EVOLUTIONARY RELATIONSHIPS AMONG MEMBERS OF THE GENUS CHLAMYDIA BASEDON 16S RIBOSOMAL DNA ANALYSIS

Nucleotide sequences from strains of the four species currently in the genus Chlamydia, C. pecorum, C. pneumoniae, C. psittaci, and C. trach omatis were investigated. In vitro-amplified RNA genes of the ribosoma l small subunit from 30 strains of C. pneumoniae and C. pecorum were s ubjected to solid-phase DNA sequencing of both strands. The human isol ates of C. pneumoniae differed in only one position in the 16S rRNA ge ne, indicating genetic homogeneity among these strains. Interestingly, horse isolate N16 of C. pneumoniae was found to be closely related to the human isolates of this species, with a 98.9% nucleotide similarit y between their 16S rRNA sequences. The type strain and koala isolates of C. pecorum were also found to be very similar to each other, posse ssing two different 16S rRNA sequences with only one-nucleotide differ ence. Furthermore, the C. pecorum strains truncated the 16S rRNA molec ule by one nucleotide compared to the molecules of the other chlamydia l species. This truncation was found to result in loss of a unilateral ly bulged nucleotide, an attribute present in all other eubacteria. Th e phylogenetic structure of the genus Chlamydia was determined by anal ysis of 16S rRNA sequences. All phylogenetic trees revealed a distinct line of descent of the family Chlamydiaceae built of two main cluster s which we denote the C. pneumoniae cluster and the C. psittaci cluste r. The clusters were verified by bootstrap analysis of the trees and s ignature nucleotide analysis. The former cluster contained the human i solates of C. pneumoniae and equine strain N16. The latter cluster con sisted of C. psittaci, C. pecorum, and C. trachomatis. The members of the C. pneumoniae cluster showed tight clustering and strain N16 is li kely to be a subspecies of C. pneumoniae since these strains also shar e some antigenic cross-reactivity and clustering of major outer membra ne protein gene sequences. C. psittaci and strain N16 branched early o ut of the respective cluster, and interestingly, their inclusion bodie s do not stain with iodine. Furthermore, they also share less reliable features like normal elementary body morphology and plasmid content. Therefore, the branching order presented here is very likely a true re flection of evolution, with strain N16 of the species C. pneumoniae an d C. psittaci forming early branches of their respective cluster and w ith C. trachomatis being the more recently evolved species within the genus Chlamydia.