EVOLUTION OF THE TRYPTOPHAN BIOSYNTHETIC-PATHWAY IN BUCHNERA (APHID ENDOSYMBIONTS) - STUDIES OF PLASMID-ASSOCIATED TRPEG WITHIN THE GENUS UROLEUCON

Aphids obtain tryptophan from prokaryotic endosymbionts assigned to th e genus Buchnera. The rate-limiting enzyme in tryptophan biosynthes, a nthranilate synthase, is encoded by the genes trpE and trpG. In Buchne ra within the family Aphididae, trpEG is plasmid-borne, apparently as an adaptation to overproduce tryptophan for hosts. To explore the evol ution of these plasmids, sequences for trpEG, the upstream region cont aining the plasmid origin of replication, and chromosomal trpB were ob tained for Buchnera of three species in the aphid genus Uroleucon and analyzed together with sequences for six other aphid species. Phylogen ies based on trpB and trpEG agree with each other and with previous vi ews of aphid phylogeny. Synonymous substitutions are about twice as hi gh for plasmid-borne genes as for chromosomal genes in the same lineag es, suggesting higher mutation rates for genes on plasmids. Nonsynonym ous rates for trpEC: are accelerated within Buchnera of Uroleucon, ind icating a change in selection intensity within this genus. Accelerated evolution within Uroleucon also seems to characterize the upstream re gion containing the putative origin of replication. (C) 1997 Academic Press.