RNA Virus Genomes

Positive-Strand RNA Viruses:

The ultimate size of single-stranded RNA genomes is limited by the fragility of RNA & the tendency of long strands to break. In addition, RNA genomes tend to have higher mutation rates than those composed of DNA because they are copied less accurately, which also tends to drive RNA viruses towards smaller genomes.

Single-stranded RNA genomes vary in size from those of Coronaviruses at approximately 30kb long to those of bacteriophages such as MS2 & Qβ at about 3.5kb. Such genomes from different virus families share a number of common features:

• Purified (+)sense vRNA is directly infectious when applied to susceptible host cells in the absence of any virus proteins (although it is about one million times less infectious than virus particles).
• There is an untranslated region (UTR) at the 5' end of the genome which does not encode any proteins & a shorter UTR at the 3' end. These regions are functionally important in virus replication & are thus conserved in spite of the pressure to reduce genome size.
• Both ends of (+)stranded eukaryotic virus genomes are often modified, the 5' end by a small, covalently attached protein or a methylated nucleotide 'cap' structure & the 3' end by polyadenylation. These signals allow vRNA to be recognised by host cells & to function as mRNA.

Negative-Strand RNA Viruses:

Viruses with negative-sense RNA genomes are a little more diverse than positive-stranded viruses. Possibly because of the difficulties of expression, they tend to have larger genomes encoding more genetic information. Because of this, segmentation is a common though not universal feature of such viruses.

Negative-sense RNA genomes are not infectious as purified RNA. This is because such virus particles all contain a virus-specific polymerase. The first event when the virus genome enters the cell is that the (-)sense genome is copied by the polymerase, forming either (+)sense transcripts which are used directly as mRNA, or a double-stranded molecule known either as the replicative intermediate (RI) or replicative form (RF), which serves as a template for further rounds of mRNA synthesis. Therefore, since purified negative-sense genomes cannot be directly translated & are not replicated in the absence of the virus polymerase, these genomes are inherently non-infectious.

Ambisense Genome Organization:

Some RNA viruses are not strictly 'negative-sense' but ambisense, since they are part (-)sense & part (+)sense:

Дата добавления: 2014-01-11