The endosymbiont (Buchnera sp.) of the aphid Diuraphis noxia contains plasmids consisting of trpEG and tandem repeats of trpEG pseudogenes

Abstract

Most aphids are dependent for their survival on prokaryotic endosymbionts assigned to the genus Buchnera. Among the functions of Buchnera species is the synthesis of tryptophan, which is required by the aphid host. In Buchnera species from the aphid Diuraphis noxia, the genes for anthranilate synthase (trpEG) were found on a plasmid which consisted of seven tandem repeats of a 3.2-kb unit and one 2.6-kb unit which differed in containing a 0.6-kb deletion. One of the 3.2-kb units contained open reading frames corresponding to trpEG; the remaining units contained trpEG pseudogenes (psi). The nucleotide sequence upstream of trpE contained a region that has characteristics of an origin of replication (ori). Relative to trpB (a chromosomal gene), there were about two copies of the trpEG-containing plasmid. Comparisons of the nucleotide sequences of the 3.2-kb units containing trpEG and psi trpEG indicated that most changes occurred in a 700-nucleotide segment that included the region upstream of trpE and the portion of this gene coding for the N terminus. The consequence of these changes was the silencing of trpEG by inactivation of the putative promoter region and premature termination of the TrpE peptide. In contrast, the nucleotide sequence of the segment corresponding to ori was conserved in the units containing trpEG and psi trpEG. We offer a number of speculations on the evolutionary pressure in this lineage which resulted in the silencing of most of trpEG while still retaining the regions resembling ori.

How nutritionally imbalanced is phloem sap for aphids?

“…Interestingly, U. sonchi shows an unusual reduction in the number of functional tryptophan biosynthetic genes (trpEG) present on symbiont plasmids; most of the copies of the genes are nonfunctional pseudogenes . A similar situation exists in Diuraphis noxia, which causes feeding damage inducing elevated levels of essential amino acids (Telang et al, in press), and which also has lost most functional tryptophan biosynthetic genes as pseudogenes (Lai et al, 1996). Thus, aphids that ingest phloem with unusually high proportions of essential amino acids may experience reduced selection for amino acid overprovisioning by symbionts, resulting in the loss of the symbionts extra copies of amino acid biosynthetic genes.…”

A bacterial genome in flux: the twelve linear and nine circular extrachromosomal DNAs in an infectious isolate of the Lyme disease spirochete Borrelia burgdorferi

“…Relatively few pseudogenes have been found in other bacteria, and these have been rare exceptions when compared with the number of functional genes. Genes with one or a few frameshifts, in frame stop codons or inactivated control regions have been found in a few anecdotal cases (for example Hall et al, 1983;Morris et al, 1995;Fsihi et al, 1996;Lai et al, 1996). The number of such genes in the completely sequenced bacterial genomes is low, e.g.…”

Phytophagous Insect–microbe Mutualisms and Adaptive Evolutionary Diversification

“…These plasmids show inter‐ and intraspecific variation in functional gene copy number and the number of amino acid biosynthesis pseudogenes (Baumann et al 1995; Thao et al 1998; Plague et al 2003; Birkle et al 2004; Moran and Degnan 2006). The observed variation in plasmid borne gene copy number among aphid lineages may be adaptive, resulting from selection favoring the functional inactivation of amino acid biosynthesis genes due to costs associated with overproduction of amino acids readily obtained from the diet (Atkinson 1977; Lai et al 1996; Wernegreen and Moran 2000). Recent studies have also linked the inactivation of chromosomal endosymbiont amino acid biosynthesis genes in various aphid lineages to variation in the nutritional content of their host plant's phloem (Tamas et al 2002; van Ham et al 2003; Moran and Degnan 2006).…”