Kinetoplast ( English kinetoplast ) is a network of molecules located in giant mitochondria and containing many copies of the mitochondrial genome [1] [2] . Most often, kinetoplast is in the form of a disk, although exceptions are also known from this rule. Kinetoplast is present only in the simplest class of kinetoplastids . Variations in the structure of the kinetoplast can reflect phylogenetic relationships within the kinetoplastids [3] . Kinetoplast is usually located near the basal body of the flagellum , therefore, it is probably firmly connected with the cytoskeleton . Kinetoplast can be easily visualized in cells using staining [4] .
Content
Structure
Kinetoplast contains DNA in two forms: and maxikolts. Maxikolts contain from 20 to 40 thousand base pairs (kilobases, kb) and are in the kinetoplast among several tens. One kinetoplast contains several thousand mini rings containing 0.5-1 kb. Maxikolts encode the proteins necessary for the functioning of the giant mitochondria in which the kinetoplast is located. The only known function of mini rings at the moment is to regulate the expression of max rings by forming guiding RNAs . Maxikings and minikings are catenated with each other, forming a flat network similar to chain mail . During cDNA replication, rings are first separated, and in daughter kinetoplasts they again catenate [4] [5] . The structure of the cDNA is best studied in , which is a catenated disk of mini- and maxicircles, most of which are not supercoiled [3] . On the outside, two protein complexes directly rotated 180 ° relative to each other and involved in the replication of mini rings [1] [2] [4] [5] are directly adjacent to the cDNA.
Different representatives of kinetoplastids kinetoplast and its DNA have a different structure. The following options are known that differ from the typical scheme described above [3] :
- pro-cDNA : kinetoplast is an elongated, bundle-like structure located in the mitochondria matrix proximal to the flagellar body. In contrast to the typical cDNA structure, only a small percentage of rings are linked to each other. At the same time, the mini and max rings are in a relaxed (not supercoiled ) state. Such a structure is described by , Bodo designis , Procryptobia sorokini syn. Bodo sorokini , Rhynchomonas nasuta, and Cephalothamnium cyclopi [3] ;
- poly-cDNA : the structure of the kinetoplast is similar to the previous version, contains few linked rings and does not contain supercoiled. A distinctive feature is that this type of kinetoplast is not a single body, but is distributed in the form of separate clusters of DNA throughout the mitochondrial matrix. This variant was found in Dimastigella trypaniformis ( commensal living in the intestines of termites ), Dismastigella mimosa (free-living kinetoplastids) and Cruzella marina ( intestinal parasite ascidium ) [3] ;
- pan-cDNA : like the previous types, contains a small percentage of linked rings, but may contain supercoiled mini rings. Pan-cDNA occupies almost the entire mitochondrial matrix , and is not scattered throughout it in the form of separate clusters. Found in Cryptobia helicis (a parasite that lives in the snail receptor ), Bodo caudatus, and Cryptobia branchialis (fish parasite);
- mega-cDNA : distributed evenly throughout the mitochondria matrix, while it does not contain minicircles. Sequences similar to those of the minikings of other kinetoplasts are tandemly combined into larger molecules with a length of about 200 kb. Mega-cDNA or structurally similar variants were found in Trypanoplasme borreli ( fish parasite) and Jarrellia sp . ( whale parasite) [3] .
Replication
Kinetoplast doubling occurs simultaneously with doubling of the neighboring flagellum immediately before the start of nuclear DNA replication. In a typical kinetoplast (as in Crithidia fasciculata ), replication is initiated when the cDNA minicircles are opened by . Free minicircles enter the space between the kinetoplast and the inner mitochondrial membrane , known as the kinetoflagellar zone [2] [3] [5] . Further, the minicircles, through an unknown mechanism, are transferred to oppositely located antipodial protein complexes that contain endonuclease , helicase , DNA polymerase , DNA primase and DNA ligase , which eliminate replication errors in newly doubled minicircles [4] . Freshly replicated minicircles can be distinguished from mature minicircles by the presence of a narrow gap. Minicircles not subjected to doubling remain covalently closed. Immediately after replication, all newly doubled minirings attach to the cDNA network, and their gaps are partially repaired [1] [5] .
As long as the mini-ring replication continues, the cDNA network continuously rotates around the central axis of the disk to prevent the addition of new mini-rings to the maternal kinetoplast. It is believed that the rotation is directly related to the doubling of the neighboring flagellum, since the daughter basal body rotates around the mother in time with the rotation of the kinetoplast. Due to the rotation, the minicircles of the daughter kinetoplast join into a spiral and gradually shift to the center of the disk, as other minicircles are cleaved from the mother cDNA and sent to the kinetoflagellar zone for replication [2] [4] [5] .
The mechanism of doubling maxikings has not been studied as thoroughly as the minikings. It was possible to identify a structure called nabelschnur (from the German " umbilical cord "), which binds the daughter cDNA with the original cDNA before separation. Using FISH, it was possible to prove that nabelschnur consists of maxikole cDNA [4] .
In the process of kinetoplast replication, five stages are distinguished, each of which is associated with the doubling of the neighboring flagellum. 1. Stage I. Kinetoplast did not begin to replicate; there are no antipodial protein complexes in it. 2. Stage II . Antipodial complexes begin to appear in the kinetoplast. The doubling of the basal body of the flagellum begins. 3. Stage III . Separation of a new flagellum begins, kinetoplast acquires a two-part appearance. 4. Stage IV . Subsidiary kinetoplasts are practically separated and only nabelschnur bind. 5. Stage V. Subsidiary kinetoplasts are finally disconnected, nabelschnur is destroyed. The structure of kinetoplasts is identical to that in the first stage [4] .
Reparation
Trypanosoma cruzi is capable of repairing nucleotides in both nuclear DNA and cDNA that were damaged by reactive oxygen species formed in the host during infection [6] . of T. cruzi cells eliminates oxidative damage to DNA by excision repair of the bases . This enzyme probably eliminates oxidative damage to cDNA caused by genotoxic stress during its replication [6] .
Notes
- ↑ 1 2 3 Shapiro TA , Englund PT The structure and replication of kinetoplast DNA. (Eng.) // Annual Review Of Microbiology. - 1995. - Vol. 49 . - P. 117-143 . - DOI : 10.1146 / annurev.mi.49.100195.001001 . - PMID 8561456 .
- ↑ 1 2 3 4 Shlomai J. The structure and replication of kinetoplast DNA. (English) // Current Molecular Medicine. - 2004 .-- September ( vol. 4 , no. 6 ). - P. 623-647 . - PMID 15357213 .
- ↑ 1 2 3 4 5 6 7 Lukes J. , Guilbride DL , Votýpka J. , Zíková A. , Benne R. , Englund PT Kinetoplast DNA network: evolution of an improbable structure. (English) // Eukaryotic Cell. - 2002 .-- August ( vol. 1 , no. 4 ). - P. 495-502 . - PMID 12455998 .
- ↑ 1 2 3 4 5 6 7 Gluenz E. , Povelones ML , Englund PT , Gull K. The Kinetoplast Duplication Cycle in Trypanosoma brucei Is Orchestrated by Cytoskeleton-Mediated Cell Morphogenesis // Molecular and Cellular Biology. - 2010 .-- 20 December ( vol. 31 , no. 5 ). - P. 1012-1021 . - ISSN 0270-7306 . - DOI : 10.1128 / MCB.01176-10 .
- ↑ 1 2 3 4 5 Torri, A., et al. DNA Replication in Eukaryotic Cells . Cold Spring Harbor Laboratory Press. 1996. pages = 1029-42. ISBN 0-87969-459-9
- ↑ 1 2 Schamber-Reis BL , Nardelli S. , Régis-Silva CG , Campos PC , Cerqueira PG , Lima SA , Franco GR , Macedo AM , Pena SD , Cazaux C. , Hoffmann JS , Motta MC , Schenkman S. , Teixeira SM , Machado CR DNA polymerase beta from Trypanosoma cruzi is involved in kinetoplast DNA replication and repair of oxidative lesions. (English) // Molecular And Biochemical Parasitology. - 2012 .-- June ( vol. 183 , no. 2 ). - P. 122-131 . - DOI : 10.1016 / j.molbiopara.2012.02.007 . - PMID 22369885 .