Biochemical pathways in prokaryotes can be traced backward through evolutionary time

For the first time, a credible prokaryotic phylogenetic tree is beingassembled by Woese and others using quantitative sequence analysis ofoligonucleotides in the highly conservative rRNA. This provides anevolutionary scale against which the evolutionary steps that led to thearrangement and regulation of contemporary biochemical pathways can bemeasured. This paper presents an emerging evolutionary picture of aromaticamino acid biosynthesis within a large superfamily assemblage ofprokaryotes that is sufficiently developed to illustrate a new perspectivethat will be applicable to many other biochemical pathways.