EVOLUTION OF AROMATIC AMINO-ACID BIOSYNTHESIS AND APPLICATION TO THE FINE-TUNED PHYLOGENETIC POSITIONING OF ENTERIC BACTERIA

A comprehensive phylogenetic tree for virtually the entire assemblage of enteric bacteria is presented. Character states of aromatic amino acid biosynthesis are used as criteria, and the results are compared with partial trees based upon sequencing of 16S rRNA, 5S rRNA, and tryptophan leader peptide. Three major clusters are apparent. Enterocluster 1 possesses a gene fusion (trpG-trpD) encoding anthranilate synthase: anthranilate 5-phosphoribosylpyrophosphate phosphoribosyltransferase of tryptophan biosynthesis. This cluster includes the genera Escherichia, Shigella, Citrobacter, Salmonella, Klebsiella, and Enterobacter. The remaining two clusters lack the trpG-trpD gene fusion, but differ in the presence (enterocluster 2) or absence (enterocluster 3) of the three-step overflow pathway to L-phenylalanine. Enterocluster 2 consists of the genera Serratia and Erwinia. Enterocluster 3 includes the genera Cedecea, Kluyvera, Edwardisella, Hafnia, Yersinia, Proteus, Providencia, and Morganella. Within these three major clusters, a tentatively hierarchy of subcluster ordering is formulated on the basis of all data available. This hierarchical framework is proposed as a general working basis for continued refinement of the phylogenetic relationships of enteric bacteria.