Plasmids in the aphid endosymbiont Buchnera aphidicola with the smallest genomes. A puzzling evolutionary story

The Striking Case of Tryptophan Provision in the Cedar Aphid Cinara cedri

“…1A). The putative origin of replication is located downstream of trpG and contains 21 direct repeats, each of which has a DnaA box in the complemen-tary strand, which is similar to the previously described B. aphidicola Trp plasmids (6,17). The trpEG genes described for other B. aphidicola strains are present either on a plasmid or on the chromosome and are always separated from the remaining genes for the pathway (10).…”

“…One contig, which is 2,795 bp long, contains the trpE and trpG genes that code for antrhanilate synthase, and the other, which is 12,480 bp long, contains the trpD, trpC, trpB, and trpA genes. The first contig had a plasmid structure, exhibiting BLASTX search homology with all the tryptophan plasmids described previously for several B. aphidicola strains, and the highest scores were associated with plasmid pTrp-BCt from Cinara tujafilina, an aphid closely related to C. cedri (6,8,17). The new plasmid was designated pTrpBCc.…”

“…All the sequences obtained for endosymbionts of aphids belonging to the subfamily Lachninae are grouped together, supporting the BLASTX search results that showed that there were high levels of homology with the proteins of B. aphidicola strains from the aphids Tuberolachnus salignus and C. tujafilina. In these strains the trpE and trpG genes are located on a pTrp plasmid and a pLeu/Trp chimeric plasmid, respectively (6). In B. aphidicola BCc no tryptophan genes are present in the main chromosome, and no tryptophan plasmid was found during genome sequencing, probably due to the method employed to obtain an enriched fraction of the B. aphidicola chromosome (for details, see reference 13).…”

What traits are carried on mobile genetic elements, and why?

“…For example, plasmids commonly carry resistance genes that enable bacteria to grow in the presence of antibiotics or heavy metals, which tend to have spatially or temporarily variable distributions (Eberhard, 1990). Plasmids also have important roles in the establishment of antagonisms or mutualisms between prokaryotes and eukaryotes, such as virulence traits (Buchrieser et al, 2000), nitrogen fixation by genes coded in rhizobia symbiotic plasmids (Nuti et al, 1979) or amino acid production in Buchnera (Nuti et al, 1979;Gil et al, 2006). Phages have been shown to be important in bacterial virulence, as in the case of cholera (Faruque et al, 2005), or in mutualisms, as in the case of an aphid-bacterial association (Oliver et al, 2009).…”

Insights into the bovine rumen plasmidome

“…Plasmids isolated thus far from different ecological niches encode a versatile array of accessory functions, ranging from antibiotic resistance to nitrogen fixation. These functions may confer an advantage to their host in its ecological niche, making the burden of carrying the plasmid worthwhile (3,(6)(7)(8)(9)(10). Therefore, accessory functions carried by plasmids in a specific ecological niche may be enriched by requirements that are relevant for that niche.…”