By far largest family of viruses is the (-) ssRNA family of viruses. Their viral RNA genome can not be directly translated instead the (-) strand is complementary to the viral mRNAs that need to be produced and translated into viral proteins. For reasons that are unclear nature has created hundreds of different (-) ssRNA viruses ranging from the measles and influenza viruses to the rabies and Ebola viruses. They are all, without exception, lipid-containing viruses
Their highlypeculiar replicative strategies would suggest a common ancestry, but this is difficult to prove given the great differences that these modern day viruses now display. We will illustrate this type of virus through the vesicular stomatitis virus (VSV). VSV is a close relative of the rabies virus. It infects horses, cattle and pigs and produces lesions on the hooves and mouths of infected animals. It can be passed to humans where it reslts in fever and lesions in the mouth. After entering a host cell, the VSV (-) strand of RNA faces a ogistical problem even greater than that faced by the polio virus. As with the poliovirus, there is no host enzyme that uses RNA as a template for nucleic acid synthesis. In addition, this (-) tranded RNA is not recognized as a template by the host ribosomes and thus this enzyme can not be directly produced
How does the virus break out of this circular trap? It does so by packaging a viral RNA- dependent RNA polymerase along with the (-) ssRNA genome within the nucleocapsid. Therefore, once inside the cell, the viral polymerase begins to work on the (-) ssRNA making two kinds of (+) stranded viral RNA. Some of this (+) stranded RNA is made as short viral mRNAs that are then transcribed into viral proteins, some of it is made as full length RNA that is then replicated to make the (-) ssRNA genome that is needed for the progeny virions
Some of the (-) ssRNA viruses have segmented genomes. For example, Ebola virus is thought to have three distinct (-) ssRNAs in it genome, each encoding a separate protein. Members of the influenza family of viruses have eight different (-) ssRNAs. The life cycle of these viruses is as described above, but each viral RNA is replicated separately, and packaging must be regulated to ensure that each virion receives one of each distinct RNA
Their highlypeculiar replicative strategies would suggest a common ancestry, but this is difficult to prove given the great differences that these modern day viruses now display. We will illustrate this type of virus through the vesicular stomatitis virus (VSV). VSV is a close relative of the rabies virus. It infects horses, cattle and pigs and produces lesions on the hooves and mouths of infected animals. It can be passed to humans where it reslts in fever and lesions in the mouth. After entering a host cell, the VSV (-) strand of RNA faces a ogistical problem even greater than that faced by the polio virus. As with the poliovirus, there is no host enzyme that uses RNA as a template for nucleic acid synthesis. In addition, this (-) tranded RNA is not recognized as a template by the host ribosomes and thus this enzyme can not be directly produced
How does the virus break out of this circular trap? It does so by packaging a viral RNA- dependent RNA polymerase along with the (-) ssRNA genome within the nucleocapsid. Therefore, once inside the cell, the viral polymerase begins to work on the (-) ssRNA making two kinds of (+) stranded viral RNA. Some of this (+) stranded RNA is made as short viral mRNAs that are then transcribed into viral proteins, some of it is made as full length RNA that is then replicated to make the (-) ssRNA genome that is needed for the progeny virions
Some of the (-) ssRNA viruses have segmented genomes. For example, Ebola virus is thought to have three distinct (-) ssRNAs in it genome, each encoding a separate protein. Members of the influenza family of viruses have eight different (-) ssRNAs. The life cycle of these viruses is as described above, but each viral RNA is replicated separately, and packaging must be regulated to ensure that each virion receives one of each distinct RNA