Evolution of the Tryptophan Biosynthetic Pathway inBuchnera(Aphid Endosymbionts): Studies of Plasmid-AssociatedtrpEGwithin the GenusUroleucon

✍ Scribed by Dadbeh Rouhbakhsh; Marta A. Clark; Linda Baumann; Nancy A. Moran; Paul Baumann

• Book ID: 102615396
• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 160 KB
• Volume: 8
• Category: Article
• ISSN: 1055-7903
• DOI: 10.1006/mpev.1997.0419

✦ Synopsis

Aphids obtain tryptophan from prokaryotic endosymbionts assigned to the genus Buchnera. The rate-limiting enzyme in tryptophan biosynthes, anthranilate synthase, is encoded by the genes trpE and trpG. In Buchnera within the family Aphididae, trpEG is plasmid-borne, apparently as an adaptation to overproduce tryptophan for hosts. To explore the evolution of these plasmids, sequences for trpEG, the upstream region containing the plasmid origin of replication, and chromosomal trpB were obtained for Buchnera of three species in the aphid genus Uroleucon and analyzed together with sequences for six other aphid species. Phylogenies based on trpB and trpEG agree with each other and with previous views of aphid phylogeny. Synonymous substitutions are about twice as high for plasmid-borne genes as for chromosomal genes in the same lineages, suggesting higher mutation rates for genes on plasmids. Nonsynonymous rates for trpEG are accelerated within Buchnera of Uroleucon, indicating a change in selection intensity within this genus. Accelerated evolution within Uroleucon also seems to characterize the upstream region containing the putative origin of replication.