Certain segments of rickettsial
genomes resemble those of
mitochondria. The deciphered genome of
R. prowazekii is 1,111,523
bp long and contains 834
genes. Unlike free-living bacteria, it contains no genes for
anaerobic glycolysis or genes involved in the biosynthesis and regulation of
amino acids and
nucleosides. In this regard, it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used.
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. 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 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.
Bacterial small RNAs 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
endothelial cells (HMEC) infected with
Rickettsia. 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. ==Naming==