Rubella virus
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Dima Nimri, M.D. [2]
Overview
Rubella virus is a single stranded, positive sense RNA virus (ssRNA). It is the only member of the genus Rubivirus belongs to the family of Togaviridae. Rubella virus is only known to infect humans, and it is responsible for causing the common childhood Rubella infection. Rubella infection is also known as German Measles or Three Day Measles. The most devastating consequence is when Rubella virus infects pregnant women during their first trimester, as it may result in congenital rubella syndrome in the newborn. Rubella infection has largely been eradicated in the developed world since the introduction of the MMR vaccine. However, it is still a challenge in many parts of the developing world due to cost and availability of the MMR vaccine.
Genome Structure
The Rubella virus genome is composed of 9762 nucleotides and encodes 2 nonstructural polypeptides (p150 and p90) within its 5′-terminal two-thirds and 3 structural polypeptides (C, E2, and E1) within its 3′-terminal one-third. Both envelope proteins E1 and E2 are glycosylated. The Rubella virus has the highest concentration of G/C nucleotides of any RNA virus, with 69.5% of the genome consisting of those nucleotides. Rubella has been sequenced completely for three strains showing >95% homology between the three strains.
There are three sites that are highly conserved in Togaviruses:
- A stem-and-loop structure at the 5' end of the genome
- A 51-nucleotide conserved sequence near the 5' end of the genome and
- A 20-nucleotide conserved sequence at the subgenomic RNA start site. Homologous sequences are present in the rubella genome.
The genome encodes several non-coding RNA structures. Among those is the rubella virus 3' cis-acting element, which contains multiple stem-loops and one of the stem-loop structures has been found to be essential for viral replication.
Cell Structure and Life Cycle
Rubella virus is an enveloped virus, circular or oval in shape and 60nm in diameter. The virion is composed of a capsid core containing a single copy of genomic RNA. The outer membrane is a lipid bilayer containing specialized glycoproteins (E1 and E2) believed to be responsible for attachment to host cells. It is also believed that a pH of 6.0 or less induces conformational changes in the glycoproteins making attachment of the viral envelope to host cells more likely. Rubella virus likely enters cells via endocytosis. Once in the cell a conformational change occurs in the capsid shell releasing the genetic information into the cell. Replication is slow with a latency period of 8-12 hours, with structural proteins appearing at 12-16 hours and peak viral 36-48 hours after infection. In volunteer subjects infected via aerosol, the characteristic rash typically appears 16-20 days from the time of exposure.