6533b853fe1ef96bd12acd0c

RESEARCH PRODUCT

Plasmid-encoded anthranilate synthase (TrpEG) in Buchnera aphidicola from aphids of the family pemphigidae

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Aphids are dependent on an intracellular symbiont (Buchnera aphidicola, Proteobacteria) for normal growth and reproduction (7, 19, 45). The bacteria reside in specialized cells in the aphid hemocele and are transmitted maternally through infection of eggs or embryos (11, 26). Phylogenetic studies have revealed two major characteristics of the evolutionary history of the association (37, 39); (i) the symbiosis had a single origin, dated about 150 million to 250 million years ago; and (ii) host and symbiont lineages have since diverged strictly in parallel. The association, like other symbioses in insects feeding on restricted and unbalanced diets, is thought to have a nutritional basis (5–7, 20). Aphids feed on plant phloem sap, a diet rich in carbohydrates but deficient in nitrogenous compounds, including most essential amino acids (16, 18, 27, 41). Buchnera has been proposed as the source of essential amino acids for the aphid (14), which has been supported by evidence from nutritional and physiological studies (17, 20–22, 45) and, more recently, by the finding of genetic modifications in the tryptophan and leucine biosynthetic pathways in Buchnera from several aphid species. In both cases, genes encoding key enzymes in the respective pathways were found to be amplified and relocated to plasmids (10, 30). Lai et al. (30) found that the genes for the two subunits of anthranilate synthase (trpE and trpG), the first enzyme in the pathway leading to tryptophan, are contained on a low-copy-number plasmid in Buchnera from the aphid Schizaphis graminum (Aphididae). The plasmid consisted of four identical tandem repeats of a 3.6-kb trpEG-containing unit. trpEG was amplified about 16-fold over the remaining genes of the pathway, which reside in a single locus [trpDC(F)BA] on the Buchnera chromosome (38). trpEG-encoding plasmids have since been found in Buchnera from various species of the Aphididae (4, 32, 42, 43), and their overall similarity suggests that the amplification is ancestral to this lineage (32). The Aphididae is the largest and evolutionarily most successful family of aphids. Many of its species have high growth and reproductive rates, and it includes a number of major agricultural pests (8). In contrast, Buchnera from the aphid Schlechtendalia chinensis, a member of the distantly related family Pemphigidae, was found to carry all the genes of the tryptophan pathway on the chromosome, organized into two single-copy linkage groups [trpEG and trpDC(F)BA] (31). This difference in organization, which is assumed to reflect a difference in the capacity to overproduce tryptophan, has been linked to potentially varying requirements for the amino acid by aphid hosts. S. chinensis has a long development time and a low reproductive rate, and its demand for tryptophan may therefore be lower than in the highly prolific aphids of the Aphididae (5–7, 9, 31). Here we report on the finding and molecular characterization of trpEG-containing plasmids in Buchnera from the aphids Tetraneura caerulescens and Pemphigus spyrothecae, both belonging to the Pemphigidae. We propose a scenario for the evolution of trp in Buchnera in which there was a single ancestral transfer of trpEG to a RepA/C-like replicon followed by independent events of replicon replacement and back-transfer of trpEG to the chromosome in different lineages.