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Researchers Sequence Smallest Genome Yet; Bacterial Endosymbiont Has Only 182 Genes

NEW YORK (GenomeWeb News) – An international research team has sequenced the genome of Carsonella ruddii, a symbiotic bacteria that has the fewest genes of any organism sequenced to date.  
The genome of C. ruddii, which lives inside certain cells in a small insect called a psyllid, has only 159,662 base pairs and 182 protein-coding genes, according to a paper published in today’s Science.
The bacteria’s genome is only one-third the size of the previously reported smallest cellular genome, the archaeal parasite Nanoarchaeum equitans, which was sequenced in 2003.
Co-author Nancy Moran, a professor at the University of Arizona, said in a statement that the small size of the organism’s genome was “very surprising.”
Moran said that even though endosymbionts generally need fewer genes to survive as compared to free-living bacteria, the researchers had not expected so few in C. ruddii. “It's believed that more genes are required for a cell to work," she said.
One explanation for the "streamlined" genome, the authors write in the Science paper, is that some C. ruddii genes may have been transferred into the host genome, enabling the insect to make some of the metabolites the bacteria needed. 

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