The tryptophan biosynthetic pathway of aphid endosymbionts (Buchnera): Genetics and evolution of plasmid-associated anthranilate synthase (trpEG) within the aphididae

Abstract

The bacterial endosymbionts (Buchnera) from the aphids Rhopalosiphum padi, R. maidis, Schizaphis graminum, and Acyrthosiphon pisum contain the genes for anthranilate synthase (trpEG) on plasmids made up of one or more 3.6-kb units. Anthranilate synthase is the first as well as the rate-limiting enzyme in the tryptophan biosynthetic pathway. The amplification of trpEG on plasmids may result in an increase of enzyme protein and overproduction of this essential amino acid, which is required by the aphid host. The nucleotide sequence of trpEG from endosymbionts of different species of aphids is highly conserved, as is an approximately 500-bp upstream DNA segment which has the characteristics of an origin of replication. Phylogenetic analyses were performed using trpE and trpG from the endosymbionts of these four aphids as well as from the endosymbiont of Schlechtendalia chinensis, in which trpEG occurs on the chromosome. The resulting phylogeny was congruent with trees derived from sequences of two chromosome-located bacterial genes (part of trpB and 16S ribosomal DNA). In turn, trees obtained from plasmid-borne and bacterial chromosome-borne sequences were congruent with the tree resulting from phylogenetic analysis of three aphid mitochondrial regions (portions of the small and large ribosomal DNA subunits, as well as cytochrome oxidase II). Congruence of trees based on genes from host mitochondria and from bacteria adds to previous support for exclusively vertical transmission of the endosymbionts within aphid lineages. Congruence with trees based on plasmid-borne genes supports the origin of the plasmid-borne trpEG from the chromosomal genes of the same lineage and the absence of subsequent plasmid exchange among endosymbionts of different species of aphids.

Vertical Transmission of Biosynthetic Plasmids in Aphid Endosymbionts ( Buchnera )

“…A recent study found congruence of gene genealogies in Buchnera of Uroleucon ambrosiae, suggesting strictly vertical transfer even within the same host species (22). These results support previous conclusions of congruence among Buchnera genealogies and contribute to the larger picture of vertical plasmid transmission across millions of years (5,6,10,48,49,52,63,64). Our data suggest that the single proposed instance of plasmid transfer in Buchnera may represent a very rare event that occurred early in the evolution of the Pemphigidae (62).…”

“…In contrast to facultative symbionts such as Rhizobium and Vibrio, lateral gene transfer in Buchnera may be highly constrained since this obligate symbiont spends its entire life cycle within specialized host cells (bacteriocytes) (11,43). In accordance with this hypothesis, several previous studies show phylogenetic congruence among chromosomal (trpB and 16S rRNA) and plasmid (trpEG and leuABCD) genes of Buchnera associated with the family Aphididae and suggest a lack of plasmid transfer in this symbiont group (5,6,10,22,48,49,51,63,64). However, recent work suggests horizontal transfer of the plasmid-encoded repA1 gene in Buchnera of Pemphigus spyrothecae (62).…”

“…In Buchnera aphidicola, the primary endosymbiont of aphids, genes for the biosynthesis of tryptophan (trpEG) and leucine (leuABCD) often occur on multicopy plasmids (pTrpEG and pLeu, respectively) (5,6,10,31,48,49,52,63,64). Comparative sequence analysis indicates that the ancestral location for both trpEG and leuABCD genes was the Buchnera chromosome, not an exogenous plasmid (7,49,63).…”

“…Comparative sequence analysis indicates that the ancestral location for both trpEG and leuABCD genes was the Buchnera chromosome, not an exogenous plasmid (7,49,63). This movement of chromosomal loci onto plasmids is considered a host-beneficial adaptation of Buchnera to overproduce these essential amino acids that are lacking in the hosts' diet of plant sap.…”

Chromosomal stasis versus plasmid plasticity in aphid endosymbiont Buchnera aphidicola

“…Extensive studies carried out on the location of the trpEG and, mainly, on the leuABCD genes in several aphid subfamilies, either on plasmids or on the main chromosome (Lai et al, 1995Rouhbakhsh et al, 1996;van Ham et al, 1997van Ham et al, , 1999van Ham et al, , 2000Sabater-Muñ oz et al, 2002, revealed a great plasticity throughout B. aphidicola evolution, showing that the genomic stasis hypothesis is not supported when plasmids are taken into account. In fact, the evolutionary history of plasmids is puzzling, since not all of the bacterial lineages carry plasmids, and not all plasmids have the same gene content and/or gene order.…”