Degenerative minimalism in the genome of a psyllid endosymbiont

Psyllids, like aphids, feed on plant phloem sap and are obligately associat ed with prokaryotic endosymbionts acquired through vertical transmission fr om an ancestral infection. We have sequenced 37 kb of DNA of the genome of Carsonella ruddii, the endosymbiont of psyllids, and found that it has a nu mber of unusual properties revealing a more extreme case of degeneration th an was previously reported from studies of eubarterial genomes, including t hat of the aphid endosymbiont Buchnera aphidicola, Among the unusual proper ties are an exceptionally low guanine-plus-cytosine content (19.9%), almost complete absence of intergenic spaces, operon fusion, and lack of the usua l promoter sequences upstream of 16S rDNA. These features suggest the synth esis of long mRNAs and translational coupling. The most extreme instances o f base compositional bias occur in the genes encoding proteins that have le ss highly conserved amino acid sequences; the guanine-plus-cytosine content of some protein-coding sequences is as low as 10%. The shift in base compo sition has a large effect on proteins: in polypeptides of C. ruddii, half o f the residues consist of five amino acids with codons low in guanine plus cytosine, Furthermore, the proteins of C. ruddii are reduced in size, with an average of about 9% fewer amino acids than in homologous proteins of rel ated bacteria, These observations suggest that the C. ruddii genome is not subject to constraints that limit the evolution of other known eubacteria.