Nasuia deltocephalinicola was recently discovered to have the smallest genome of all bacteria, with 112,091 nucleotides. For comparison, the human genome has 3.2 billion nucleotides. The second smallest genome, from bacteria Tremblaya princeps, has 139,000 nucleotides. While Nasuia deltocephalinicola has the smallest number of nucleotides it has more protein-coding genes than some bacteria, with 137 protein-coding genes.
Nasuia deltocephalinicola was discovered when leafhoppers and other phloem and xylem-feeding insects were investigated for endosymbiotic bacteria. The phloem and xylem of plants are rich in carbohydrates, in the form of sucrose, but lack lipids and proteins. Lipids can be synthesized from carbohydrates; however, proteins require nitrogen, which is not commonly found in plant sap. Nasuia deltocephalinicola, with the help of other bacterial endosymbionts, help the insects by synthesizing 10 essential amino acids that the insects would not otherwise have. The only insects that can benefit from this relationship are those from the suborder Sternorrhyncha, which feed off phloem, and those from the suborder Auchenorrhyncha, which feed off xylem. Nasuia deltocephalinicola can synthesize two of the essential amino acids that these insects require. Nasuia deltocephalinicola uses the UGA codon in its DNA to specify tryptophan instead of the stop as in most other organisms.
The symbiotic relationship between Nasuia deltocephalinicola and leafhoppers is proposed to have started at least 200 million years ago, when leafhoppers and spittlebugs evolutionarily diverged. This claim is supported by the fact that Nasuia deltocephalinicola's closest bacterial relative is Zinderia insecticola, which plays the same role for spittlebugs as Nasuia deltocephalinicola plays for leaf hoppers. Leafhoppers return the favor to bacteria like Nasuia deltocephalinicola by providing shelter in the form of a specialized organ in their abdominal cavity called a bacteriome, which they have on both sides of their abdomens. Many types of bacteria can reside in these organs, though the bacteria are completely separated from each other and reside in different sections of the bacteriome.
Nasuia deltocephalinicola is an obligate endosymbiont, as it cannot thrive without being in a leafhopper. One of the reasons for this is that Nasuia deltocephalinicola no longer uses the genes needed to synthesize ATP through oxidative phosphorylation. It is proposed that this is because of the high sucrose concentration found in xylem and phloem of plants.
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