Mimivirus is a genus of giant viruses, in the family Mimiviridae. Amoeba serve as their natural hosts.[2][3] This genus contains a single identified species named Acanthamoeba polyphaga mimivirus (APMV). It also refers to a group of phylogenetically related large viruses.[4]
In colloquial speech, APMV is more commonly referred to as just "mimivirus". Mimivirus, short for "mimicking microbe", is so called to reflect its large size and apparent Gram-staining properties.[5]
Mimivirus has a large and complex genome compared with most other viruses. Until 2013, when a larger virus Pandoravirus was described, it had the largest capsid diameter of all known viruses.[6]
APMV was discovered accidentally in 1992 within the amoeba Acanthamoeba polyphaga, after which it is named, during research into legionellosis by researchers from Marseille and Leeds.[7] The virus was observed in a Gram stain and mistakenly thought to be a Gram-positive bacterium. As a consequence it was named Bradfordcoccus, after Bradford, England, where the amoeba had originated. In 2003, researchers at the Université de la Méditerranée in Marseille, France, published a paper in Science identifying the micro-organism as a virus. It was given the name "mimivirus" (for "mimicking microbe") as it resembles a bacterium on Gram staining.[8]
The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it.[9]
Mimivirus has been placed into a viral family by the International Committee on Taxonomy of Viruses as a member of the Mimiviridae,[10] and has been placed into Group I of the Baltimore classification system.[11]
Although not strictly a method of classification, mimivirus joins a group of large viruses known as nucleocytoplasmic large DNA viruses (NCLDV). They are all large viruses which share both molecular characteristics and large genomes. The mimivirus genome also possesses 21 genes encoding homologs to proteins which are seen to be highly conserved in the majority of NCLDVs, and further work suggests that mimivirus is an early divergent of the general NCLDV group.[8]
The mimivirus is the fourth-largest virus, preceded by the recently discovered Megavirus chilensis, Pandoravirus and Pithovirus. Mimivirus has a capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface of the capsid, bringing the total length of the virus up to 600 nm. Variation in scientific literature renders these figures as highly approximate, with the "size" of the virion being casually listed as anywhere between 400 nm and 800 nm, depending on whether total length or capsid diameter is actually quoted.
Its capsid appears hexagonal under an electron microscope, therefore the capsid symmetry is icosahedral.[12] It does not appear to possess an outer viral envelope, suggesting that the virus does not exit the host cell by exocytosis.[13] Mimivirus shares several morphological characteristics with all members of the NCLDV group of viruses. The condensed central core of the virion appears as a dark region under the electron microscope. The large genome of the virus resides within this area. An internal lipid layer surrounding the central core is present in all other NCLDV viruses, so this features may also be present in mimivirus.[12]
Several mRNA transcripts can be recovered from purified virions. Like other NCLDVs, transcripts for DNA polymerase, a capsid protein and a TFII-like transcription factor were found. However, three distinct aminoacyl tRNA synthetase enzyme transcripts and four unknown mRNA molecules specific to mimivirus were also found. These pre-packaged transcripts can be translated without viral gene expression and are likely to be necessary to Mimivirus for replication. Other DNA viruses, such as the Human cytomegalovirus and Herpes simplex virus type-1, also feature pre-packaged mRNA transcripts.[13]
The mimivirus genome is a linear, double-stranded molecule of DNA with 1,181,404 base pairs in length.[14] This makes it one of the largest viral genomes known, outstripping the next-largest virus genome of the Cafeteria roenbergensis virus by about 450,000 base pairs. In addition, it is larger than at least 30 cellular clades.[15]
In addition to the large size of the genome, mimivirus possesses an estimated 979 protein-coding genes, far exceeding the minimum 4 genes required for viruses to exist (c.f. MS2 and Qβ viruses).[16] Analysis of its genome revealed the presence of genes not seen in any other viruses, including aminoacyl tRNA synthetases, and other genes previously thought only to be encoded by cellular organisms. Like other large DNA viruses, mimivirus contains several genes for sugar, lipid and amino acid metabolism, as well as some metabolic genes not found in any other virus.[13] Roughly 90% of the genome was of coding capacity, with the other 10% being "junk DNA".
The stages of mimivirus replication are not well known, but as a minimum it is known that mimivirus attaches to a chemical receptor on the surface of an amoeba cell and is taken into the cell. Once inside, an eclipse phase begins, in which the virus disappears and all appears normal within the cell. After about 4 hours small accumulations can be seen in areas of the cell. 8 hours after infection many mimivirus virions are clearly visible within the cell. The cell cytoplasm continues to fill with newly synthesised virions, and about 24 hours after initial infection the cell likely bursts open to release the new mimivirus virions.[13]
Little is known about the details of this replication cycle, most obviously attachment to the cell surface and entry, viral core release, DNA replication, transcription, translation, assembly and release of progeny virions. However, scientists have established the general overview given above using electron micrographs of infected cells. These micrographs show mimivirus capsid assembly in the nucleus, acquisition of an inner lipid membrane via budding from the nucleus, and particles similar to those found in many other viruses, including all NCLDV members. These particles are known in other viruses as viral factories and allow efficient viral assembly by modifying large areas of the host cell.
Mimivirus may be a causative agent of some forms of pneumonia; this is based mainly on indirect evidence in the form of antibodies to the virus discovered in pneumonia patients.[17] However, the classification of mimivirus as a pathogen is tenuous at present as there have been only a couple of papers published potentially linking mimivirus to actual cases of pneumonia. A significant fraction of pneumonia cases are of unknown cause,[18] though a mimivirus has been isolated from a Tunisian woman suffering from pneumonia.[19] There is evidence that mimivirus can infect macrophages.[20]
Mimivirus shows many characteristics which place it at the boundary between living and non-living. It is as large as several bacterial species, such as Rickettsia conorii and Tropheryma whipplei, possesses a genomic size comparable to that of several bacteria, including those above, and codes for products previously not thought to be encoded by viruses (including a kind of collagen[21]). In addition, mimivirus has genes coding for nucleotide and amino acid synthesis, which even some small obligate intracellular bacteria lack. They do, however, lack any genes for ribosomal proteins, making mimivirus dependent on a host cell for protein translation and energy metabolism.[21]
Because its lineage is very old and could have emerged prior to cellular organisms,[22][23] Mimivirus has added to the debate over the origins of life. Some genes that code for characteristics unique to Mimivirus, including those coding for the capsid, have been conserved in a variety of viruses which infect organisms from all domains. This has been used to suggest that Mimivirus is related to a type of DNA virus that emerged before cellular organisms and played a key role in the development of all life on Earth.[22] An alternative hypothesis is that there were three distinct types of DNA viruses that were involved in generating the three known domains of life—eukarya, archaea and bacteria.[23] It has been suggested that Mimivirus and similar kinds are remnants of a "fourth domain" of life, and that other giant virus may represent other ancient domains.[21]
Nevertheless, mimivirus does not exhibit the following characteristics, all of which are part of many conventional definitions of life:
Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License. Mimivirus is a genus of giant viruses, in the family Mimiviridae. Amoeba serve as their natural hosts. This genus contains a single identified species named Acanthamoeba polyphaga mimivirus (APMV). It also refers to a group of phylogenetically related large viruses.
In colloquial speech, APMV is more commonly referred to as just "mimivirus". Mimivirus, short for "mimicking microbe", is so called to reflect its large size and apparent Gram-staining properties.
Mimivirus has a large and complex genome compared with most other viruses. Until 2013, when a larger virus Pandoravirus was described, it had the largest capsid diameter of all known viruses.
Mimivirus est un genre de virus à ADN. Les virus de ce taxon sont particulièrement grands, souvent plus gros que bien des bactéries, et de ce fait on les qualifie de girus — techniquement ils en sont le holotype. Au niveau écologique, Mimivirus infecte une amibe : Acanthamoeba polyphaga.
En 2009, son origine est sujette à discussion parmi les biologistes et certains voient en lui le représentant d'une nouvelle branche de l'arbre phylogénétique[1], donnant ainsi un argument supplémentaire en faveur des théories selon lesquelles les virus devraient être considérés comme des êtres vivants à part entière[2].
Jusqu'en 2012, Mimivirus était le quatrième plus grand virus connu — les premiers étant le virus ebola (dont la longueur peut dépasser 1 000 nm), Mamavirus[3], et Megavirus chilensis[4]. En 2013, une équipe de chercheurs marseillais découvre les Pandoravirus, soit deux virus encore plus gros et génétiquement plus complexes que certaines bactéries, dans des sédiments en mer au Chili, et en eau douce en Australie[5].
Parce qu'anciennement les biologistes et virologues n'imaginaient pas qu'un virus puisse surclasser en dimensions une bactérie, la découverte de la nature virale d'un parasite de Acanthamoeba polyphaga allait prendre plus de dix ans.
C'est en 1992 qu'est prélevée l'eau d'une tour de climatisation industrielle à Bradford, en Angleterre. Y sont identifiés Acanthamoeba polyphaga et la découverte d'un parasite nouveau : d'abord assimilé à une bactérie, les « cellules » sont dénommées Bradford coccus.
Ce n'est qu'en 2003 que sera identifié Mimivirus en tant que virus à l'université de la Méditerranée à Marseille[6].
La séquence complète de son génome a été publiée en 2004[7].
La description taxonomique est publiée en 2005 sous l'égide de l'Union Internationale des Sociétés Microbiologiques et de sa Division Virologie[8].
Officiellement, le professeur de microbiologie Didier Raoult a baptisé ce microbe du nom de Mimivirus parce que cela signifie « Mimicking Microbe Virus ».
Officieusement, ce serait en souvenir des aventures de « Mimi l'amibe », un héros de son enfance sorti de l'imagination de son père qui lui racontait de la sorte l'histoire de l'évolution[9].
À côté de toutes ses spécificités, Mimivirus tend à confirmer certains aspects de la définition d'un virus donnée par André Lwoff, notamment l'absence de ribosome et de ses protéines et l'incapacité de se diviser[10]. Son mode de réplication passe par une phagocytose du virion, puis après une phase d'éclipse la mise en place d'usine à virions.
De forme icosaédrique, il mesure 400 nanomètres de diamètre. Le matériel viral est introduit à son apex avant terminaison de la synthèse de la capside, alors que le vertex en forme d'étoile à cinq branches fusionnera avec la membrane du phagosome pour relarguer son contenu dans le cytoplasme hôte.
En plus de son ADN, il contient également de l'ARN et partage ainsi cette particularité avec les cytomégalovirus, mettant à mal ce dogme dans la définition des virus[10].
Le génome contient environ 1 181 404 paires de bases et 1 262 gènes[11]. C'était deux fois plus gros que les plus imposants virus identifiés à l'époque, plus gros que celui des plus petites bactéries[1].
Chose étrange, certains de ces gènes (une trentaine) ne sont pas présents chez les autres virus, mais le sont dans les organismes cellulaires, comme ceux codant des protéines de réparation de l'ADN ou de la traduction de l'ARN en protéines. Ces gènes ne devraient pas servir à Mimivirus, car les virus utilisent la machinerie cellulaire de leur hôte[10]. À tel point que certains ont relié Mimivirus à l'arbre de la vie aux côtés des trois domaines du vivant que sont les archées, les bactéries et les eucaryotes[1].
Ces affinités avec le vivant ne font pas pour autant de lui un virus vraiment à part comparé aux autres virus, car il partage des dizaines de gènes spécifiques aux virus et neuf gènes qui sont communs à l'ensemble des autres virus[10].
Son rôle comme agent de la pneumonie chez l'humain a été rapporté après la contamination d'un technicien de laboratoire[12]. Un modèle expérimental a montré qu'il pouvait provoquer des pneumonies chez la souris.
L'étude de Mimivirus par l'équipe de Didier Raoult a permis de découvrir en août 2008 un nouveau type de virus, surnommé Spoutnik. Ce dernier a la particularité de ne pas pouvoir infecter de cellule, ce qui le rend inapte à se multiplier par la seule méthode alors connue chez les virus. Pour accomplir son cycle de réplication, Spoutnik infecte une forme particulièrement grosse de Mimivirus, désignée sous le nom de Mamavirus, et détourne la machinerie cellulaire de la cellule-hôte, l'amibe Acanthamoeba polyphaga, que le Mimivirus a lui-même détournée. Ce nouveau type de virus parasitant d'autres virus a été dénommé virophage par analogie avec le terme bactériophage désignant les virus infectant les bactéries. Cette découverte est une première et démontre la richesse et la complexité de ces particules aux frontières du vivant[13].
Ce virus n'est en 2008 pas classé par l'International Committee on Taxonomy of Viruses (ICTV)[14]. Mais cette non-classification n'empêche cependant pas de le classer dans celle de Baltimore, où sa structure nucléique le range dans le groupe I des virus à ADN bicaténaires.
Par sa structure et la conservation de certains gènes spécifiques aux grands virus nucléocytoplasmiques (NCLDV), le mimivirus est classé dans ce groupe, mais il n'a cependant d'affinité particulière avec aucune des cinq familles de ce groupe, ce qui pousse certains virologistes à le classer au sein d'une nouvelle famille, les Mimiviridae[1].
Mimivirus est un genre de virus à ADN. Les virus de ce taxon sont particulièrement grands, souvent plus gros que bien des bactéries, et de ce fait on les qualifie de girus — techniquement ils en sont le holotype. Au niveau écologique, Mimivirus infecte une amibe : Acanthamoeba polyphaga.
En 2009, son origine est sujette à discussion parmi les biologistes et certains voient en lui le représentant d'une nouvelle branche de l'arbre phylogénétique, donnant ainsi un argument supplémentaire en faveur des théories selon lesquelles les virus devraient être considérés comme des êtres vivants à part entière.
Jusqu'en 2012, Mimivirus était le quatrième plus grand virus connu — les premiers étant le virus ebola (dont la longueur peut dépasser 1 000 nm), Mamavirus, et Megavirus chilensis. En 2013, une équipe de chercheurs marseillais découvre les Pandoravirus, soit deux virus encore plus gros et génétiquement plus complexes que certaines bactéries, dans des sédiments en mer au Chili, et en eau douce en Australie.
미미바이러스(Mimivirus)는 바이러스의 일종으로, 미미바이러스과에 속한다. 또한, 이것은 세포생리학적으로 관련된 큰 바이러스의 집단을 가리킨다. 구어체로, APMV는 흔히 미미바이러스라고 불리기도 한다. 모방 미생물의 줄임말인 미미바이러스는 바이러스의 큰 크기와 그람 자국의 성질을 반영하여 그렇게 불린다. 또한, 미미바이러스는 다른 바이러스들보다 더 복잡한 유전자를 가지고 있다. 더 큰 바이러스인 판도라바이러스가 설명된 2013년까지 알려진 바이러스 들 중 캡시드 지름이 가장 컸다.
APMV는 1992년 마르세유와 리즈의 연구자들이 레지오넬라증을 연구하던 도중에 아메바 아칸타아메바 폴리파가에서 우연히 발견되었다. 이 바이러스는 그람 염색을 통해 관찰되었고, 그람 양성 박테리아로 잘못 생각되었다. 결과적으로 이것은 아메바가 발견된 영국의 브래드퍼드 대학교의 이름을 따 브래드퍼드코커스라고 명명되었다. 2003년에 프랑스 마르세유에 있는 유니버스테 데 라 메디테라네(Université de la Méditerranée)의 연구원들이 미생물을 바이러스로 인식하는 논문을 사이언스지에 발포하였다. 미미바이러스는 그람 염색의 박테리아의 모습을 닮아서 "미생물을 닮았다"는 뜻의 미미바이러스라고 명명되었다. 미미바이러스를 발견한 연구 팀은 나중에 마마바이러스로 불리는 조금 더 큰 바이러스와 그것을 감염시키는 스푸트니크 바이러스도 발견하였다.
미미바이러스 국제 바이러스 분류 위원회에 의해 미미바이러스과의 일원으로 바이러스계에 편입되어 볼티모어 분류 체계 I조에 편입되었다. 엄격하게 분류하는 방법은 아니지만 미미바이러스는 핵세포질 대 바이러스(NCLDV)로 알려진 거대 바이러스 그룹에 속하며 모두 분자의 특성과 큰 유전자들을 공유한다. 또한, 미미바이러스 유전자는 대부분의 NCLDV에서 보존도가 높은 그룹의 초기 분립자임이 밝혀졌다.
이 미미바이러스는 메가바이러스 칠렌시스, 판도라바이러스, 피토바이러스에 이어서 4번째로 큰 바이러스이다. 미미바이러스의 캡시드 직경은 400 nm 정도이다. 캡시드 표면에서 100 nm 크기의 단백질 필라멘트는 바이러스의 총길이를 600nm까지 키운다. 과학 문헌의 발달은 이러한 수치를 매우 가깝게 제시하는데 전체 길이 또는 캡시드 직경이 실제로 인정되는지 여부에 따라서 바이러스의 총 길이가 400 nm에서 800 nm 사이로 에상된다. 그것의 캡시드는 전자 현미경아래에 육각형으로 나타나기 때문에 캡시드 대칭은 20면체이다. 또한, 외부 바이러스 막을 가지고 있는 것처럼 보이지 않으며, 바이러스가 난세포에 의해 숙주 세포에서 빠져나가지 않는다는 것을 알 수 있다.
미미바이러스는 NCLDV 바이러스 그룹의 구성원과 몇 가지의 형태학적 특성들을 공유한다. 비리온의 응축된 중심핵은 전자 현미경에서 어두운 영역으로 나타나며 바이러스의 큰 유전자들은 대부분 이 지역에 서식한다. 중심핵을 둘러싸고 있는 내부의 지질층이 다른 모든 NCLDV 바이러스에도 존재하기 때문에 이 특징 또한 미미바이러스에도 존재할 수 있다. 또한, 정제된 바이러스로부터 여러 mRNA 정보를 복구할 수 있다. 다른 NCLDV와 마찬가지로 DNA 중합효소, 캡시드 단백질, TFII 유사 전사계수의 정보가 발견되었다. 그러나 3개의 구별되는 아미노아실 tRNA 합성 효소 정보와 미미바이러스에 특유한 4개의 알려지지 않은 mRNA 분자도 발견되었다. 미리 포장된 이 정보들은 바이러스의 유전자 발현 없이도 번식이 가능하며 복제에 미미바이러스가 필요할 가능성이 높다. 인간 사이토메갈로 바이러스, 헤르페스 심플렉스 바이러스-1도 미리 포장된 mRNA 기록이 특징이다.
미미바이러스 - 네이버 지식백과
