SARS-related coronavirus

Severe acute respiratory syndrome–related coronavirus (SARSr-CoV or SARS-CoV, Betacoronavirus pandemicum) is a species of virus consisting of many known strains. Two strains of the virus have caused outbreaks of severe respiratory diseases in humans: severe acute respiratory syndrome coronavirus 1 (SARS-CoV or SARS-CoV-1), the cause of the 2002–2004 outbreak of severe acute respiratory syndrome (SARS), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the pandemic of COVID-19. There are hundreds of other strains of SARSr-CoV, which are only known to infect non-human mammal species: bats are a major reservoir of many strains of SARSr-CoV; several strains have been identified in Himalayan palm civets, which were likely ancestors of SARS-CoV-1.

These enveloped, positive-sense single-stranded RNA viruses enter host cells by binding to the angiotensin-converting enzyme 2 (ACE2) receptor. The SARSr-CoV species is a member of the genus Betacoronavirus and the only species of the subgenus Sarbecovirus (SARS Betacoronavirus).

The SARS-related coronavirus was one of several viruses identified by the World Health Organization (WHO) in 2016 as a likely cause of a future epidemic in a new plan developed after the Ebola epidemic for urgent research and development before and during an epidemic towards diagnostic tests, vaccines and medicines. This prediction came to pass with the COVID-19 pandemic.

SARS-related coronavirus is a member of the genus Betacoronavirus (group 2) and monotypic of the subgenus Sarbecovirus (subgroup B). Sarbecoviruses, unlike embecoviruses or alphacoronaviruses, have only one papain-like proteinase (PLpro) instead of two in the open reading frame ORF1ab. SARSr-CoV was determined to be an early split-off from the betacoronaviruses based on a set of conserved domains that it shares with the group.

Bats serve as the main host reservoir species for the SARS-related coronaviruses like SARS-CoV-1 and SARS-CoV-2. The virus has coevolved in the bat host reservoir over a long period of time. Only recently have strains of SARS-related coronavirus been observed to have evolved into having been able to make the cross-species jump from bats to humans, as in the case of the strains SARS-CoV-1 and SARS-CoV-2. Both of these strains descended from a single ancestor but made the cross-species jump into humans separately. SARS-CoV-2 is not a direct descendant of SARS-CoV-1.

The SARS-related coronavirus is an enveloped, positive-sense, single-stranded RNA virus. Its genome is about 30 kb, which is one of the largest among RNA viruses. The virus has 14 open reading frames which overlap in some cases. The genome has the usual 5′ methylated cap and a 3′ polyadenylated tail. There are 265 nucleotides in the 5'UTR and 342 nucleotides in the 3'UTR.

The 5' methylated cap and 3' polyadenylated tail allows the positive-sense RNA genome to be directly translated by the host cell's ribosome on viral entry. SARSr-CoV is similar to other coronaviruses in that its genome expression starts with translation by the host cell's ribosomes of its initial two large overlapping open reading frames (ORFs), 1a and 1b, both of which produce polyproteins.

The functions of several of the viral proteins are known. ORFs 1a and 1b encode the replicase/transcriptase polyprotein, and later ORFs 2, 4, 5, and 9a encode, respectively, the four major structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N). The later ORFs also encode for eight unique proteins (orf3a to orf9b), known as the accessory proteins, many with no known homologues. The different functions of the accessory proteins are not well understood.