Candidatus Carsonella ruddii

Candidatus Carsonella ruddii

Classification

Higher Order Taxa: Bacteria (Domain); Proteobacteria (Phylum); Gamma proteobacteria (Class); Unclassified Gamma proteobacteria (Order); Candidatus Carsonella (Genus); C. C. ruddii (Species)

Species: Candidatus Carsonella ruddii

C. C. ruddii got its name from well-known naturalists. Carsonella came from a revolutionary American environmentalist, Rachel Carson, best known for her groundbreaking work decades ago, Silent Sping; ruddii is named after another American naturalist, Robert L. Rudd.

Description and Significance

C. C. ruddii is a gram-negative, primary obligate symbiont of psyllids, particularly Pachpsylla venusta. This microorganism has the smallest genome known. There is much controversy over whether this is a living cell or simply an organelle as it is missing genes needed for living independently.

Genome Structure

C. C. ruddii has the smallest known cellular genome. The circular genome was finished being sequenced on October 21, 2006 and was found to be only 159,662 base pairs. Because of the genome size, 93% is coding including 213 genes with 182 of those being protein coding. These genes tend to overlap and are smaller than what is expected of bacteria. Out of the 7% non-coding, there are no pseudogenes and contains no phage sequences. It has a GC content of 19.9%, which is extremely low. This low GC content concludes an AT bias which is not generally observed in their free living counterparts. It is likely due to deleterious mutations caused by genetic drift because of small population sizes and not much recombination. Because there are so few genes, there were doubts if C. C. ruddii was an actual living cell. It has been determined that is a living organism simply in a symbiotic relationship with its host, and the genes that are coded for by the organism are those necessary for essential living functions and those that are beneficial to the host’s fitness. The C. C. ruddii genome appears to be missing some important genes such as that for gyrase, ligase, RNAse HI, as well as histone-like and single stranded-binding proteins; making its genome insufficient to replicate, transcribe and synthesize proteins. The genes for helicase and primase are too degraded for use. The gene believed to be the signal factor subunit is also highly degraded, making the only available transcription machinery that of the core subunits of the RNA polymerase. The translation machinery is also affected, being limited by only minimal RNA genes, three rRNA and 28 tRNA genes, that are required for protein synthesis. The ability to make working ribosomes is also under question since 9 aminoacyl-tRNA synthetases and 15 ribosomal protein components are not functional or completely missing, as well as proteins required for ribosome maturation. Some translation factors are also missing like elongation factors P and T. Although many genes are missing, C. C. ruddii still has genes to carry out reproduction and maintenance, which are necessary functions for life.

Cell Structure and Metabolism

As an obligate endosymbiont to Pachypsylla venusta, C. C. ruddii must rely on its host. The psyllids feed on plant sap, which lacks certain nutrients. The endosymbiont provides essential nutrients the host needs and is not getting from the plant sap. This relationship works so well that in studies when antibiotics are used to rid the insect of that endosymbiont, this yields devastating outcomes for the development, reproduction, and survival of the insect.

Ecology

C. C. ruddii live inside the body cavity of psyllids, insects which feed on plant sap. They are found in specialized host cells called bacteriocytes. A collection of bacteriocytes makes a bacteriome. The symbiosis is mutually beneficial as the microorganism receives food and basic living from the insect and the insect receives amino acids it is not capable of making itself.

Pathology

C. C. ruddii is an obligate endosymbiont to the insects psyllids, particularly Pachypsylla venusta. However, they are not known to be disease-causing. Application to Biotechnology C. C. ruddii is an endosymbiont, therefore disruption of this organism could lead to its use as a potential method of pest control (controlling psyllid populations). This may be a reason for its being named after naturalists who both wrote important books on pesticides and the environment.

Current Research

The most recent study was in 2007, on the subject of whether the microorganism was an actual living cell or an organelle. Also, it has played a significant part in a study of genomic analysis. Relying on its characteristic low GC content, scientists were able to find a new approach of isolating DNA of endosymbionts. Usually, isolating endosymbiont DNA is difficult because of the low amount DNA from the microorganism and the especially high possibility for host DNA contamination. Tried methods included density gradient centrifugation and dissecting out the endosymbionts. Endosymbionts and bacteria that live in mixed populations are difficult to isolate for DNA analysis. This new method was devised to recover endosymbiont in an effective way.

References

"Carsonella ruddii." Uniprot. 9 Dec 2008 <http://www.uniprot.org/taxonomy/114186>. Dale, Colin. "Extracting single genomes from heterogenous DNA samples." PubMed. 5 Dec 2008 <http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1283884>. Douglas, Angela. "Symbiotic Microorganisms: Untapped Resources for Insect Pest Control." Science Direct. 18 June 2007. 5 Dec 2008 <http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCW-4P0N8TS- 2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1 &_urlVersion=0&_userid=10&md5=f135b32a2e635cd117752f0c05c8078b>. "Genome Project Result." NCBI. 9 Dec 2008 <http://www.ncbi.nlm.nih.gov/sites/entrez?db=genomeprj&cmd=Retrieve&dopt=Overview&list _uids=17977>. Moran, et al. "The players in a mutualistic symbiosis: Insects, bacteria, viruses, and virulence genes." Proc Natl Acad Sci USA. 22 November 2005. <http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1287993> Minkel, JR. "Tiny Genome May Reflect Organelle in the Making." Scientific America. 12 October 2006. 9 Dec 2008 <http://www.sciam.com/article.cfm?id=tiny-genome-may-reflect-o>. Nakabachi, Atsushi. "Researchers Find Smallest Cellular Genome." Innovations Report. 13 October 2006. 9 Dec 2008 <http://www.innovations-report.com/html/reports/life_sciences/report- 72018.html>. Tamames, Javier. "The Frontier Between Cell and Organelle." BMC Evolutionary Biology. 2007. PubMed Central. 5 Dec 2008 <http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2175510>. Thao, MyLo. "Cospeciation of Psyllids and Their Primary Prokaryotic Endosymbionts." AEM. 5 Dec 2008 <http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=138992>. Yamishita, A. "Smallest Ever Genome Sequenced." Riken Research. 5 Dec 2008 <http://www.rikenresearch.riken.jp/research/126/>. Zeintz, Evelyn. "Genome Independenc in Insect-Bacterium Symbiosis ." PubMed. 5 Dec 2008 <http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=138992>.