Candidatus Carsonella ruddii

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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 two 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 (jumping plant lice), 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 sequenced to date. The circular genome was finished on October 21, 2006 by groups in Japan and the University of Arizona. It was found to be only 159,662 base pairs (Tamames) compared this with the genome of Mycoplasma genitalium, the smallest free-living genome, with about 580,000 bp. An estimated 93-97% of the genome is coding, including 213 genes with 182 of these coding for protein. These genes tend to overlap (much more than is common in bacteria) and are smaller than what is expected of bacteria in general. The remaining 7% non-coding part contains no pseudogenes and no phage sequences. The C. C. ruddii genome has a 19.9% GC content, which is extremely low. This results in a significant AT bias which is not generally observed in their free living counterparts. This bias is likely due to deleterious mutations caused by genetic drift because of small population sizes and not much recombination. (Tamames) Because there are so few genes, there is debate as to whether C. C. ruddii is an actual living cell. It has been determined that it is a living organism simply in a symbiotic relationship with its host. The organism has apparently retained only those genes necessary for essential living functions and those that are beneficial to the host’s fitness. However, 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 to only minimal RNA genes, three rRNA and 28 tRNA genes, which are required for protein synthesis. The ability to make working ribosomes is also under question since nine aminoacyl-tRNA synthetases and 15 ribosomal protein components, as well as proteins required for ribosome maturation, are either not functional or completely missing. C. C. ruddii also lacks genes involved in energy metabolism and membrane synthesis. Some translation factors, such as elongation factors P and T, are also missing. Although many genes are missing, C. C. ruddii does retain genes to carry out reproduction and cellular maintenance, which are necessary functions for life. (Tamames)

Cell Structure and Metabolism As an obligate endosymbiont to Pachypsylla venusta, C. C. ruddii relies on its host for survival, and the insect relies on its endosymbionts as well. Since the psyllids feed exclusively on plant sap, which lacks many important nutrients, the endosymbiont provides essential nutrients needed by the host. This symbiotic relationship works so well that the use of antibiotics to rid the insect of its endosymbionts 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 shelter from the insect and the insect receives amino acids and other nutrients it is not getting from its diet, or is not capable of synthesizing for 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 for pest control (controlling psyllid populations). This may be one of the reasons for its being named after naturalists who both wrote important books on pesticides and the environment.

Current Research Much of the research on this organism centers on the question of whether it is a true bacteria or an organelle. Hencem studying this organism may give insights into the process of endosymbiosis and allow us to examine the steps that led to the formation of mitochondria and chloroplasts in eukaryotic cells. 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 to isolating DNA of endosymbionts. Usually, isolating endosymbiont DNA is difficult because of the low amount of 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.

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