Editing Rickettsia (section)

==Genomics==
Certain segments of rickettsial [[genome]]s resemble those of [[Mitochondrial DNA|mitochondria]].<ref name=Emelyanov_2003>{{cite journal | vauthors = Emelyanov VV | title = Mitochondrial connection to the origin of the eukaryotic cell | journal = European Journal of Biochemistry | volume = 270 | issue = 8 | pages = 1599–1618 | date = April 2003 | pmid = 12694174 | doi = 10.1046/j.1432-1033.2003.03499.x | doi-access = free }}</ref> The deciphered genome of ''R. prowazekii''  is 1,111,523 [[base pair|bp]] long and contains 834 [[genes]].<ref name=Andersson_1998>{{cite journal | vauthors = Andersson SG, Zomorodipour A, Andersson JO, Sicheritz-Pontén T, Alsmark UC, Podowski RM, Näslund AK, Eriksson AS, Winkler HH, Kurland CG | display-authors = 6 | title = The genome sequence of Rickettsia prowazekii and the origin of mitochondria | journal = Nature | volume = 396 | issue = 6707 | pages = 133–140 | date = November 1998 | pmid = 9823893 | doi = 10.1038/24094 | doi-access = free | bibcode = 1998Natur.396..133A | author1-link = Siv G. E. Andersson }}</ref> Unlike free-living bacteria, it contains no genes for [[Anaerobic respiration|anaerobic]] [[glycolysis]] or genes involved in the biosynthesis and regulation of [[amino acid]]s and [[nucleoside]]s. In this regard, it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used.

[[Adenosine triphosphate|ATP]] production in ''Rickettsia'' is the same as that in mitochondria. In fact, of all the microbes known, the ''Rickettsia'' is probably the closest relative (in a [[phylogenetic]] sense) to the mitochondria. Unlike the latter, the genome of ''R. prowazekii'', however, contains a complete set of genes encoding for the [[tricarboxylic acid cycle]] and the [[respiratory chain]] complex.  Still, the genomes of the ''Rickettsia'', as well as the mitochondria, are frequently said to be "small, highly derived products of several types of reductive evolution".

The recent discovery of another parallel between ''Rickettsia'' and viruses may become a basis for fighting [[HIV]] infection.<ref>{{cite journal | vauthors = Kannangara S, DeSimone JA, Pomerantz RJ | title = Attenuation of HIV-1 infection by other microbial agents | journal = The Journal of Infectious Diseases | volume = 192 | issue = 6 | pages = 1003–1009 | date = September 2005 | pmid = 16107952 | doi = 10.1086/432767 | doi-access = free }}</ref> Human immune response to the [[scrub typhus]] pathogen, ''[[Orientia tsutsugamushi]]'', appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. A probable reason for this actively studied phenomenon is a certain degree of [[homology (biology)|homology]] between the rickettsiae and the virus, namely, common [[epitope]](s) due to common genome fragment(s) in both pathogens. Surprisingly, the other infection reported to be likely to provide the same effect (decrease in viral load) is the virus-caused illness [[dengue fever]].

Comparative analysis of genomic sequences have also identified five [[conserved signature indels]] in important proteins, which are uniquely found in members of the genus ''Rickettsia''. These indels consist of a four-amino-acid insertion in [[transcription repair coupling factor]] Mfd, a 10-amino-acid insertion in ribosomal protein L19, a one-amino-acid insertion in [[FtsZ]], a one-amino-acid insertion in major [[sigma factor]] 70, and a one-amino-acid deletion in [[exonuclease VII]]. These indels are all characteristic of the genus and serve as molecular markers for ''Rickettsia''.<ref>{{cite journal | vauthors = Gupta RS | title = Protein signatures distinctive of alpha proteobacteria and its subgroups and a model for alpha-proteobacterial evolution | journal = Critical Reviews in Microbiology | volume = 31 | issue = 2 | pages = 101–135 | date = January 2005 | pmid = 15986834 | doi = 10.1080/10408410590922393 | s2cid = 30170035 }}</ref>

[[Bacterial small RNA]]s play critical roles in virulence and stress/adaptation responses. Although their specific functions have not been discovered in ''Rickettsia'', few studies showed the expression of novel sRNA in human microvascular [[Endothelium|endothelial cells]] (HMEC) infected with ''Rickettsia''.<ref>{{cite journal | vauthors = Schroeder CL, Narra HP, Rojas M, Sahni A, Patel J, Khanipov K, Wood TG, Fofanov Y, Sahni SK | display-authors = 6 | title = Bacterial small RNAs in the Genus Rickettsia | journal = BMC Genomics | volume = 16 | pages = 1075 | date = December 2015 | pmid = 26679185 | pmc = 4683814 | doi = 10.1186/s12864-015-2293-7 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Schroeder CL, Narra HP, Sahni A, Rojas M, Khanipov K, Patel J, Shah R, Fofanov Y, Sahni SK | display-authors = 6 | title = Identification and Characterization of Novel Small RNAs in Rickettsia prowazekii | journal = Frontiers in Microbiology | volume = 7 | pages = 859 | date = 2016 | pmid = 27375581 | pmc = 4896933 | doi = 10.3389/fmicb.2016.00859 | doi-access = free }}</ref>

Genomes of intracellular or parasitic bacteria undergo massive reduction compared to their free-living relatives. Examples include Rickettsia for alpha proteobacteria, T. whipplei for Actinobacteria, Mycoplasma for Firmicutes (the low G+C content Gram-positive), and Wigglesworthia and Buchnera for gamma proteobacteria.<ref>{{cite journal | vauthors = Raoult D, Ogata H, Audic S, Robert C, Suhre K, Drancourt M, Claverie JM | title = Tropheryma whipplei Twist: a human pathogenic Actinobacteria with a reduced genome | journal = Genome Research | volume = 13 | issue = 8 | pages = 1800–1809 | date = August 2003 | pmid = 12902375 | pmc = 403771 | doi = 10.1101/gr.1474603 }}</ref>