Large-scale label-free quantitative proteomics of the pea aphid-Buchnera symbiosis

Many insects are nutritionally dependent on symbiotic microorganisms that have tiny genomes and are housed in specialized host cells called bacteriocytes. The obligate symbiosis between the pea aphid Acyrthosiphon pisum and the γ-proteobacterium Buchnera aphidicola (only 584 predicted proteins) is particularly amenable for molecular analysis because the genomes of both partners have been sequenced. To better define the symbiotic relationship between this aphid and Buchnera, we used large-scale, high accuracy tandem mass spectrometry (nanoLC-LTQ-Orbtrap) to identify aphid and Buchnera proteins in the whole aphid body, purified bacteriocytes, isolated Buchnera cells and the residual bacteriocyte fraction. More than 1900 aphid and 400 Buchnera proteins were identified. All enzymes in amino acid metabolism annotated in the Buchnera genome were detected, reflecting the high (68%) coverage of the proteome and supporting the core function of Buchnera in the aphid symbiosis. Transporters mediating the transport of predicted metabolites were present in the bacteriocyte. Label-free spectral counting combined with hierarchical clustering, allowed to define the quantitative distribution of a subset of these proteins across both symbiotic partners, yielding no evidence for the selective transfer of protein among the partners in either direction. This is the first quantitative proteome analysis of bacteriocyte symbiosis, providing a wealth of information about molecular function of both the host cell and bacterial symbiont.

Bacterial genes in the aphid genome: absence of functional gene transfer from Buchnera to its host

Genome reduction is typical of obligate symbionts. In cellular organelles, this reduction partly reflects transfer of ancestral bacterial genes to the host genome, but little is known about gene transfer in other obligate symbioses. Aphids harbor anciently acquired obligate mutualists, Buchnera aphidicola (Gammaproteobacteria), which have highly reduced genomes (420–650 kb), raising the possibility of gene transfer from ancestral Buchnera to the aphid genome. In addition, aphids often harbor other bacteria that also are potential sources of transferred genes. Previous limited sampling of genes expressed in bacteriocytes, the specialized cells that harbor Buchnera, revealed that aphids acquired at least two genes from bacteria. The newly sequenced genome of the pea aphid, Acyrthosiphon pisum, presents the first opportunity for a complete inventory of genes transferred from bacteria to the host genome in the context of an ancient obligate symbiosis. Computational screening of the entire A. pisum genome, followed by phylogenetic and experimental analyses, provided strong support for the transfer of 12 genes or gene fragments from bacteria to the aphid genome: three LD–carboxypeptidases (LdcA1, LdcA2,ψLdcA), five rare lipoprotein As (RlpA1-5), N-acetylmuramoyl-L-alanine amidase (AmiD), 1,4-beta-N-acetylmuramidase (bLys), DNA polymerase III alpha chain (ψDnaE), and ATP synthase delta chain (ψAtpH). Buchnera was the apparent source of two highly truncated pseudogenes (ψDnaE and ψAtpH). Most other transferred genes were closely related to genes from relatives of Wolbachia (Alphaproteobacteria). At least eight of the transferred genes (LdcA1, AmiD, RlpA1-5, bLys) appear to be functional, and expression of seven (LdcA1, AmiD, RlpA1-5) are highly upregulated in bacteriocytes. The LdcAs and RlpAs appear to have been duplicated after transfer. Our results excluded the hypothesis that genome reduction in Buchnera has been accompanied by gene transfer to the host nuclear genome, but suggest that aphids utilize a set of duplicated genes acquired from other bacteria in the context of the Buchnera–aphid mutualism.

Bacterial lineages have repeatedly evolved intimate symbioses with eukaryotic hosts, the most famous cases being those of the cell organelles, mitochondria, and plastids. Symbiont genomes typically lose many ancestral genes, raising the question of how they function with so few genes. In organelles, part of the answer involves gene transfer to the host genome, allowing maintenance of essential functions. So far, the extent of gene transfer to hosts has not been assessed for other cases of intimate, obligate symbiosis. Aphids harbor an ancient coevolved intracellular symbiont, called Buchnera. We used the newly available sequence of the pea aphid genome to conduct an exhaustive computational search for genes of bacterial ancestry. We found that no functional genes have been transferred from Buchnera, ruling out such transfer as a driving force in genome reduction in this symbiont. However, the aphid genome does contain eight transcribed genes of apparent bacterial origin, some of which have been duplicated after transfer. Based on their expression patterns, most of these appear to function specifically in the aphid-Buchnera symbiosis, presenting the possibility that the maintenance of obligate intracellular symbioses can be affected by the acquisition and duplication of genes transferred from unrelated bacterial lineages.

Interactions of chaperonin with a weakly active anthranilate synthase from the aphid endosymbiont Buchnera aphidicola

The endosymbiotic bacterium Buchnera provides its aphid host with essential amino acids. Buchnera is typical of intracellular symbiotic and parasitic microorganisms in having a small effective population size, which is believed to accelerate genetic drift and reduce the stability of gene products. It is hypothesized that Buchnera mitigates protein instability with an increased production of the chaperonins GroESL. In this paper, we report the expression and functional analysis of trpE, a plasmid-borne fast-evolving gene encoding the tryptophan biosynthesis enzyme anthranilate synthase. We overcame the problem of low enzyme stability by using an anthranilate synthase-deficient mutant of E. coli as the expression host and the method of genetic complementation for detection of the enzyme activity. We showed that the Buchnera anthranilate synthase was only weakly active at the temperature of 26 degrees C but became inactive at the higher temperatures of 32 degrees C and 37 degrees C and that the coexpression with chaperonin genes groESL of E. coli enhanced the function of the Buchnera enzyme. These findings are consistent with the proposed role of groESL in the Buchnera-aphid symbiosis.

Chromosomal stasis versus plasmid plasticity in aphid endosymbiont Buchnera aphidicola

The study of three genomes of the aphid endosymbiont Buchnera aphidicola has revealed an extraordinary stasis: conservation of gene order and genetic composition of the chromosome, while the chromosome size and number of genes has reduced. The reduction in genome size appears to be ongoing since some lineages we now know to have even smaller chromosomes than the first B. aphidicola analysed. The current sequencing by our group of one of these smaller genomes with an estimated size of 450 kb, and its comparison with the other three available genomes provide insights into the nature of processes involved in shrinkage. We discuss whether B. aphidicola might be driven to extinction and be replaced by secondary aphid endosymbionts. In some lineages, genes encoding key enzymes in the pathways leading to tryptophan and leucine biosynthesis (trpEG and leuABCD, respectively) are located on plasmids, rather than the main chromosome. In contrast to the stasis of the main chromosome, plasmid genes have frequently been transferred to the main chromosome and undergone other gene rearrangements. We propose a two-step scenario to explain these contrasting modes of evolution. Essential genes may have escaped regulation by moving to plasmids in a moving B. aphidicola ancestor. B. aphidicola became polyploidy at a given stage of its evolution and plasmid genes have been transferred to the main chromosome through several independent events.

Prephenate dehydratase from the aphid endosymbiont (Buchnera) displays changes in the regulatory domain that suggest its desensitization to inhibition by phenylalanine

Buchnera aphidicola, the prokaryotic endosymbiont of aphids, complements dietary deficiencies with the synthesis and provision of several essential amino acids. We have cloned and sequenced a region of the genome of B. aphidicola isolated from Acyrthosiphon pisum which includes the two-domain aroQ/pheA gene. This gene encodes the bifunctional chorismate mutase-prephenate dehydratase protein, which plays a central role in L-phenylalanine biosynthesis. Two changes involved in the overproduction of this amino acid have been detected. First, the absence of an attenuator region suggests a constitutive expression of this gene. Second, the regulatory domain of the Buchnera prephenate dehydratase shows changes in the ESRP sequence, which is involved in the allosteric binding of phenylalanine and is strongly conserved in prephenate dehydratase proteins from practically all known organisms. These changes suggest the desensitization of the enzyme to inhibition by phenylalanine and would permit the bacterial endosymbiont to overproduce phenylalanine.