Negative ssRNA Viruses

In negative-sense single-stranded RNA (−ssRNA) viruses, the genome is complementary to mRNA. It cannot be read by ribosomes, so it must first be copied into positive-sense mRNA before any viral protein can be made.Host cells have no enzyme that can do this. The virus therefore carries its own RNA-dependent RNA polymerase inside the virus particle, packaged with the genome. Without this enzyme, the virus could not start an infection.The stages of replication are:1. Attachment and entry: the enveloped virus binds to receptors and enters the cell, and the genome is released along with its polymerase.
2. Transcription: the viral polymerase uses the − strand as a template to make + strand mRNA.
3. Translation: host ribosomes translate the mRNA into viral proteins, including capsid proteins, envelope glycoproteins and more polymerase.
4. Genome copying: the polymerase makes full-length + strands, which are then used as templates for many new − strand genomes.
5. Assembly and release: new genomes are packaged with polymerase, and the virions bud from the cell membrane, gaining their envelope.−RNA → +mRNA → proteins; −RNA → +RNA → many −RNABecause the RNA alone cannot be translated, purified naked RNA from a −ssRNA virus is not infectious. This is the key difference from +ssRNA viruses.Examples of −ssRNA viruses include influenza virus, measles virus, mumps virus, rabies virus and Ebola virus. Most are enveloped.