Carsonella ruddii

Candidatus Carsonella ruddii is an endosymbiotic gamma proteobacterium.Spaulding A.W. and C.D. von Dohlen. 1998. Phylogenetic characterization and molecular evolution of bacterial endosymbionts in Psyllids (Hemiptera: Sternorrhyncha). Molecular Biology and Evolution 15(11):1506-1513 It has the smallest known genome of any bacterium.

The species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids.Thao M.L. 2000. Cospeciation of Psyllids and their primary prokaryotic endosymbionts. Applied and Environmental Microbiology 66:2898-2905Thao M.L. 2001. Phylogenetic analysis of vertically transmitted psyllid endosymbionts (Candidatus Carsonella ruddii) based on atpAGD and rpoC: comparisons with 16S-23S rDNA-derived phylogeny. Current Microbiology 42:419-21 PMID 11381334 The endosymbionts occurs in a specialised structure known as the bacteriome.

In 2006 the genome of one strain of Ca. C. ruddi was sequenced at RIKEN in Japan and the University of Arizona. The genome is a circular chromosome of 159,662 base pairs and has a high coding density (97%) with many overlapping genes and reduced gene length. The number of predicted genes was 182, also the lowest on record (NCBI-Genome). Numerous genes considered essential for life seem to be missing, suggesting that the species may have achieved organelle-like status.

In comparison, Mycoplasma genitalium, which has the smallest genome of any free-living organism,'Free-living' in the sense of not being an endosymbiont. has a genome of 521 genes.

References 

 Other websites 
Scientific American – Tiny genome may reflect organelle in the making

Category:Gram-negative bacteria