106, and the S20w is 83-121S.Taxonomic Structure of the Family
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Family |
Microviridae |
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Genus |
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Genus |
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Genus |
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Genus |
Virions exhibit icosahedral symmetry (T = 1) with projections at each of the 12 vertices. There is no envelope, and the diameter of unstained hydrated particles is 22 nm between the depressions at the 2-fold axes and 33 nm between the outermost edges of the projections at the 5-fold axes. Thus, reported diameters from electron micrographs of negative stained preparations vary from 26-32 nm, depending on the orientation chosen for measurement (Fig. 1).
Physicochemical and Physical Properties
Virion buoyant density in CsCl is 1.36-1.41 g/cm3, depending on the genus. Infectivity is chloroform and detergent resistant and stable in the pH range of 6-9, but highly sensitive to radiation. Virion Mr (genus Microvirus) is 6-7 106, and the S20w is 83-121S.
Virions contain one molecule of circular, positive sense ssDNA; genome sizes are as follows:
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Genus |
Phage |
Number of nts |
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Microvirus |
X174 |
5,386 |
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St-1 |
6,050 | |
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Spiromicrovirus |
Sp-4 |
4,421 |
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Bdellomicrovirus |
MAC-1 |
about 4,600 |
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Chlamydiamicrovirus |
Ch-1 |
4,877 |
Several genes are translated from overlapping reading frames. The complete sequences of the genomes of X174, Enterobacteria phage S13 (S13), Enterobacteria phage G4 (G4), Enterobacteria phage 3 (3), and Enterobacteria phage K (K) (genus Microvirus), Spiroplasma phage 4 (Sp-4) (genus Spiromicrovirus), and Chlamydia phage 1 (Ch-1) (genus Chlamydiamicrovirus), are available from either GenBank or EMBL database.
Virions (genus Microvirus) contain 60 copies of three proteins (gp J, F, and G) with Mr of 4, 48, and 19 103, respectively, and 12 copies of one protein (gp H) with an Mr of 34 103. The atomic structure of X174 virus has been determined; its F capsid and G “spike” proteins each contain an eight-stranded antiparallel beta barrel similar to that found in picornaviruses and many icosahedral plant viruses. The C-terminal end of each J protein is bound to the inner surface of an F protein near the 3-fold axis, forming a binding pocket for segments of the ssDNA. The N-terminal end of J is bound to the inner surface of a neighboring F protein, cross-linking the capsid proteins into pentameric units. Seven nonstructural proteins have been identified, two of which (gp B and D with Mr of 14 and 17 103) are found in the preformed procapsid. The C-terminal ends of the 60 copies of the B morphogenic protein are bound to the inner surface of F capsid proteins in the procapsid at the same position as J in the virion. The 240 copies of the mainly alpha helical D protein form a fenestrated scaffolding shell on the outer surface of the F capsid proteins in the procapsid. The gp G and H containing “spikes” in the procapsid protrude through the openings at the 5-fold axes of the scaffold shell. The other nonstructural proteins and their functions are: gp A and C - synthesis of RF and progeny DNA, gp A* - suppression of host DNA synthesis, gp E - cell lysis, and gp K - enhance progeny yield.
The virions of Ch-1 (genus Chlamydiamicrovirus) contain only three proteins, VP1, VP2, and VP3, with predicted Mr of 67, 28, and 17 103, respectively. Interestingly, the virions of Sp-4 (genus Spiromicrovirus) have only a single capsid protein with Mr of 64 103.
None reported.
None reported.
Genome Organization and Replication
Virus replication begins with transfer of the parental ssDNA into the cell and its conversion to ds RF by host cell proteins. Messenger RNA is transcribed from this template by host cell RNA polymerase. One of the proteins (protein A) made from this mRNA becomes covalently bound to the parental RF and leads to progeny RF replication. Progeny RF molecules act as templates for mRNA synthesis and further RF replication (via ssDNA intermediates formed by rolling circle replication), until sufficient levels of viral structural proteins (F, G, and H) and two additional procapsid proteins (B and D) are made and assembled into procapsids. Procapsids then bind to some RF molecules and C protein switches DNA synthesis from ds RF replication to synthesis of progeny ssDNA. As nascent viral ssDNA is synthesized, it interacts with the J packaging proteins and is packaged into procapsids, presumably through one of the 3.0 nm openings at the 3-fold axes. At the same time the J proteins displace the B proteins, which exit through the remaining 3.0 nm openings. Less maturation of filled procapsids involves loss of the D scaffolding proteins and closing of the 3.0 nm openings in the procapsid as the cell is lysed by E protein. Since intracellular ds RF has been detected for other genera, they presumably follow a replication pathway similar to that of the genus Microvirus, but there are only limited data on the replication details of other genera of the family Microviridae (Fig. 2).
Native virions (genus Microvirus) generate both non-neutralizing and neutralizing monoclonal antibodies. Polyclonal antisera produce first-order inactivation kinetics. Members of the genus Microvirus can be assigned to at least three main groups based on serologic cross-reactivity patterns.
The host range of member viruses (genus Microvirus) is determined by the carbohydrate structure of the host cell outer membrane lipopolysaccharide receptor. Thus, various species of the Enterobacteriaceae constitute the host range for individual viruses. Similar specificity determinants are presumed for the MAC-1 and Ch-1 phage (genera Bdellomicrovirus and Chlamydiamicrovirus), which also infect Gram-negative eubacteria (genera Bdellovibrio and Chlamydia, respectively); but the nature of the specificity determinants for the genus Spiromicrovirus, which infects wall-less Spiroplasma, is not known.
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