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Journal of Bacteriology, March 2001, p. 1853-1861, Vol. 183, No. 6
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.6.1853-1861.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Degenerative Minimalism in the Genome of a Psyllid Endosymbiont

Marta A. Clark,1 Linda Baumann,1 MyLo Ly Thao,1 Nancy A. Moran,2 and Paul Baumann1,*

Microbiology Section, University of California, Davis, California 95616-8665,1 and Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 857212

Received 25 September 2000/Accepted 14 December 2000

Psyllids, like aphids, feed on plant phloem sap and are obligately associated with prokaryotic endosymbionts acquired through vertical transmission from 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 number of unusual properties revealing a more extreme case of degeneration than was previously reported from studies of eubacterial genomes, including that of the aphid endosymbiont Buchnera aphidicola. Among the unusual properties are an exceptionally low guanine-plus-cytosine content (19.9%), almost complete absence of intergenic spaces, operon fusion, and lack of the usual promoter sequences upstream of 16S rDNA. These features suggest the synthesis of long mRNAs and translational coupling. The most extreme instances of base compositional bias occur in the genes encoding proteins that have less highly conserved amino acid sequences; the guanine-plus-cytosine content of some protein-coding sequences is as low as 10%. The shift in base composition has a large effect on proteins: in polypeptides of C. ruddii, half of 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 related bacteria. These observations suggest that the C. ruddii genome is not subject to constraints that limit the evolution of other known eubacteria.

* Corrosponding author. Mailing address: Microbiology Section, University of California, One Shields Ave., Davis, CA 95616-8655. Phone: (530) 752-0272. Fax: (530) 752-9014. E-mail: pabaumann{at}ucdavis.edu.

Journal of Bacteriology, March 2001, p. 1853-1861, Vol. 183, No. 6
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.6.1853-1861.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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