4.8.1: Positive-Strand RNA Viruses of Animals

Positive-Strand RNA Viruses of Animals

Positive strand RNA viruses are the single largest group of RNA viruses with 30 families.

LEARNING OBJECTIVES

Categorize the characteristics of positive-strand viruses

KEY TAKEAWAYS

Key Points

• Nucleic acid is usually single-stranded RNA (ssRNA), but may be double-stranded RNA (dsRNA).
• An RNA virus is a virus that has RNA (ribonucleic acid) as its genetic material.
• Notable human diseases caused by RNA viruses include SARS, influenza, hepatitis C, West Nile fever, and polio.

Key Terms

• virus: A submicroscopic infectious organism, now understood to be a non-cellular structure consisting of a core of DNA or RNA surrounded by a protein coat. It requires a living cell to replicate, and often causes disease in the host organism.
• genetic: Relating to genetics or genes.
• RNA: Ribonucleic acid (RNA) is a ubiquitous family of large biological molecules that performs multiple vital roles in the coding, decoding, regulation, and expression of genes.

Single stranded RNA viruses can be classified according to the sense or polarity of their RNA into negative-sense and positive-sense, or ambisense RNA viruses. Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. As such, purified RNA of a positive-sense virus can directly cause infection though it may be less infectious than the whole virus particle. Purified RNA of a negative-sense virus is not infectious by itself as it needs to be transcribed into positive-sense RNA; each virion can be transcribed to several positive-sense RNAs. Ambisense RNA viruses resemble negative-sense RNA viruses, except they also translate genes from the positive strand. A common viral positive-strand RNA viruses that infect humans are the picornaviruses.

Figure: Foot and Mouth Disease: Foot and Mouth Disease is caused by the Aphthovirus virus which positive-strand RNA virus, of the Picornaviridae family of animal viruses.

A picornavirus is a virus belonging to the family Picornaviridae. Picornaviruses are non-enveloped, positive-stranded RNA viruses with an icosahedral capsid. The genome RNA is unusual because it has a protein on the 5′ end that is used as a primer for transcription by RNA polymerase. The name is derived from pico, meaning small, and RNA, referring to the ribonucleic acid genome, so “picornavirus” literally means small RNA virus. Picornaviruses are separated into a number of genera and include many important pathogens of humans and animals. The diseases they cause are varied, ranging from acute “common-cold”-like illnesses, to poliomyelitis, to chronic infections in livestock. Additional species not belonging to any of the recognized genera continue to be described.

Picornaviruses are separated into a number of genera. Contained within the picornavirus family are many organisms of importance as vertebrate and human pathogens, shown in the table below.Enteroviruses infect the enteric tract, which is reflected in their name. On the other hand, rhinoviruses infect primarily the nose and the throat. Enteroviruses replicate at 37°C, whereas rhinoviruses grow better at 33°C, as this is the lower temperature of the nose. Enteroviruses are stable under acid conditions and thus they are able to survive exposure to gastric acid. In contrast, rhinoviruses are acid-labile (inactivated or destroyed by low pH conditions) and that is the reason why rhinovirus infections are restricted to the nose and throat.