Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 289

Rotaviruses, the most important cause of gastroenteritis in children, are large icosahedral viruses made of a three-shelled capsid containing 11 double-stranded RNA segments. The structure of this virus indicated that a large portion of the viral genome (~25%) is ordered within the particle and forms a dodecahedral structure (see Fig. 4.19). In this structure, the RNA molecules interact with the inner capsid layer and pack around the RNA polymerase located at the fivefold axis of symmetry. Further analysis of rotavirus particles in the process of synthesizing mRNA has shown that newly synthesized molecules are extruded from the capsid through several channels located at the fivefold axes (see the figure). Multiple mRNAs are released at the same time from such particles. On the basis of these observations, it has been suggested that each double-stranded genomic RNA segment is copied by an RNA polymerase located at a fivefold axis of symmetry. This model may explain why no double-stranded RNA virus with more than 12 genomic segments, the maximal number of fivefold axes, has been found.

Three-dimensional visualization of mRNA release from rotavirus particles synthesizing mRNA. Structure of a rotavirus particle in the process of synthesizing mRNA. Parts of newly synthesized mRNA that are ordered, and therefore structurally visible, are shown in magenta at the fivefold axes of symmetry. The ordered portions of the mRNAs have been extended (gray tubes) to visualize exit from the capsid. Red circles depict mRNA cap structures. Courtesy of B. V. V. Prasad and Liya Hu, Baylor College of Medicine.

 Lawton JA, Estes MK, Prasad BV. 1997. Three-dimensional visualization of mRNA release from actively transcribing rotavirus particles. Nat Struct Biol 4:118–121.

Figure 6.25 Hepatitis delta virus RNA synthesis. (A) Schematic of the forms of hepatitis delta virus RNA and δ antigen found in infected cells. aa, amino acids; ORF, open reading frame. (B) Overview of hepatitis delta virus mRNA and genomic RNA synthesis. In steps 1 to 3, RNA synthesis is initiated by host RNA polymerase II at the indicated position on the (−) strand genomic RNA. The polymerase passes the poly(A) signal (purple box) and the self-cleavage domain (red circle). In steps 4 and 5, the 5′ portion of this RNA is processed by cellular enzymes to produce delta mRNA with a 3′ poly(A) tail, while RNA synthesis continues beyond the cleavage site and the RNA undergoes self-cleavage (step 6). RNA synthesis continues until at least one unit of the (−) strand genomic RNA template is copied. The poly(A) signal is ignored in this full-length (+) strand. In steps 7 to 10, after self-cleavage to release a full-length (+) strand, self-ligation produces a (+) strand circular RNA. In steps 11 to 20, mRNA synthesis initiates on the full-length (+) strand to produce (−) strands by a rolling-circle mechanism. Unit-length genomes are released by the viral ribozyme (step 15) and self-ligated to form (−) strand circular genomic RNAs. Data from Taylor JM. 1999. Curr Top Microbiol Immunol 239:107–122.