The genomes of all cellular organisms, from bacteria to humans, consist of double-stranded DNA. But in viruses, there is tremendous diversity of virus genomes: double stranded or single-stranded, DNA or RNA, positive- or negative-sense, but only viruses have RNA genomes.
In terms of virus genomes, “negative sense” means that a single-stranded nucleic acid molecule has the opposite sequence to messenger RNA (mRNA) and so cannot be translated into protein until it has been copied. This has important biological implications for viruses with negative-sense RNA genomes. Since cells have no biochemical mechanism to copy RNA, every negative-sense RNA virus must carry within the virus particle an RNA-dependent RNA polymerase (or “replicase” as it is frequently called), or the virus genome will be biologically meaningless once in a host cell.
There are seven virus families and 31 genera of viruses with negative-stranded RNA genomes, and these groups contain some very important pathogens. Based on their similar genetic structure, four of these virus families are believed to have arisen from a common ancestor, and are grouped into a taxonomic order, the Mononegvirales (unsegmented negative-strand viruses).
The Bornaviruses are a relatively little-studied group, giving rise to Borna disease, a neurological syndrome of warm-blooded animals. The Filoviruses have pleiomorphic (variably shaped), elongated particles approximately 80 nm in diameter and between 130-14,000 nm long – hence their name, which means “thread-like” viruses. The Filovirus genome encodes seven proteins on monocistronic mRNAs which are complementary to vRNA (the virus genome). Until recently, relatively little work has been performed on these viruses because of the difficulties of working with them, but their replication is known to be similar to that of rhabdoviruses and paramyxoviruses (also members of the Mononegvirales).
The Paramyxoviruses have enveloped particles which are 125-250nm in diameter. Their genome contains a linear arrangement of six genes, separated by repeated sequences. Paramyxoviruses include:
- Parainfluenzaviruses: These cause acute respiratory infections ranging from relatively mild influenza-like illness to bronchitis, croup and pneumonia.
- Respiratory Syncytial Virus (RSV): A major cause of lower respiratory tract disease in infants.
- Measles: A highly infectious virus spread by aerosols, which causes a systemic infection with complications including ear infections (1 in 20 cases), pneumonia (1 in 25), convulsions (1 in 200), meningitis/encephalitis (1 in 1,000), subacute sclerosing panencephalitis (SSPE) (1 in 1,000,000), and even death (1 in 2,500-5,000 cases).
The Rhaboviruses have unique bullet-shaped particles with prominent protein spikes on the surface of their lipid envelope. Rhabdovirus genomes are around 11 kilobases long and contain five genes. Diseases caused by Rhabdoviruses include vesicular stomatitis in cattle, pigs, horses and wildlife, and rabies, which causes a fatal encephalitis.
In addition to the Mononegvirales, other negative-sense RNA viruses have segmented genomes, i.e. their genomes comprise a number of separate molecules, all of which must be packaged into a particle in order to give rise to an infectious virus.
The best known of these are the Orthomyxoviruses, which include influenza virus. Influenza viruses can infect a wide variety of mammals, including humans, horses, pigs, ferrets and birds, and are a major human pathogen. Unlike other negative-sense RNA viruses, Orthomyxovirus genomes are replicated in the nucleus of the host cell, rather than in the cytoplasm.
In summary, this is possibly the most biologically diverse class of viruses.
- Mononegvirales (replication in cytoplasm)
- Orthomyxoviruses (replication in nucleus)
- Arenaviruses, Bunyaviruses (ambisense genome)
- Different strategies of gene expression to cope with genome coding patterns.
- They include some of the most important virus pathogens.