Mutualism as revealed by the genome sequence of Buchnera, an ebdocellular bacterial symbiont of insects
Shuji Shigenobu
University of Tokyo
Buchnera sp. APS is an endocellular bacterial symbiont harbored by pea aphids, Acyrthosiphon pisum. Buchnera belongs to the _-Proteobacteria, closely related to tbe Escherichia coli. Buchnera is maternally transmitted to eggs and embryos through host generations, and the mutualism between Buchnera and its host is so obligate that neither can reproduce independently. The first for symbiotic bacteria, we determined the complete genome sequence of Buchnera, by the whole genome random sequencing method. The genome of Buchnera sp. APS is comprised of one 640,681 base pair chromosome and two small plasmids, representing the smallest of the complete genomes sequenced to date except for that of Mycoplasma genitalium, which is regarded as a genome of the minimal gene set. Our analysis of the genome revealed the characteristic gene repertory of Buchnera, which is quite different from those of free-living and parasitic bacteria, reflecting mutualistic symbiosis. The gene set exhibits the give-and-take relationship between the host and the symbiont at the genomic level. For example, there are genes for the biosyntheses of essential amino acids for the hosts in the genome, whereas those for nonessential amino acids are missing, suggesting complementarity and syntrophy between the host and the symbiont. Another feature of the gene repertory of Buchnera is vulnerability. Buchnera lacks genes for cell surface components, including outer membrane proteins and lipopolysaccharides. This bacterium also lacks the moiety of DNA repair genes involved in uvr excision repair system and SOS system, indicating that Buchnera is a defenseless organism. Moreover Buchnera seems to even owe its membrane bilayer to the host, according to the genome data. Thus, Buchnera is completely symbiotic and viable only in its limited niche, bacteriocytes.
Our evolutionary analysis shows that this symbiotic gene repertoire is the consequence of the reductive evolution of the genome caused by the prolonged intracellular life. By analogy with mitochondria, Buchnera may exhibit the features of a "semi-organelle" specialized in nutrients provision for the host.
Our next interest is how the symbiosis has been operated by the limited gene repertory of Buchnera. Our ortholog analysis indicated only 4 ORFs are unique to Buchnera. Generally, the most similar counterparts of Buchnera proteins are those of E. coli, and in most cases the gene order in E. coli operons is well conserved in Buchnera. Considering these observations, the Buchnera genome is a subset of the E. coli genome, and this indicates that Buchnera has not obtained special genes for the operation of the symbiosis. Then, what is a key molecule for keeping symbiosis? There are two possible answers: (i) some genes evolved specifically in Buchnera and were endowed with new features responsible for the symbiosis. (ii) the genes encoded in the host's genome are play roles in symbiosis. We are now trying to identify the key molecules for the symbiosis, using both computational and experimental approaches, and we found some candidates of such proteins including ompF-like porin.
