One of the families of ssRNA viruses which cause upper respiratory tract diseases in humans and birds is the Coronaviridae family. Although the coronaviruses are known since the early 60s (229 and OC43 viruses), they started attracting attention only since 2003, when a virus causing a severe acute respiratory syndrome (SARS) was discovered in an outbreak that took place in southern China and then spread into several countries.1 Ten years after this event, which was responsible of 774 deaths in over 30 countries (of 8000 confirmed cases), a new “SARS-like” infection emerged from the Middle East and was named as Middle East respiratory syndrome coronavirus (MERS-CoV) by the ...view middle of the document...
Electron microscopy has shown that the virions are approximately 120nm in diameter and the ring of S protein spikes is 20nm deep. In those coronavirus where the HE protein is present, it is presented as a 5-10nm deep layer. Many species have its S protein cleaved in two subunits, the N-terminal S1 end and the C-terminal S2 sequence. The S protein has a transmembrane region in S2 which is responsible for its attaching to the envelope. The functional S protein exists as a trimer. S1 forms the outer part of the mature protein and the stalk is formed largely by S2. The variability of the S protein differs, being S1 the most variable segment reaching 40% difference between different serotypes of IBV and allowing the outsmarting of the immune system induced by other strains.4
Genome organization and expression
The genome of coronaviruses is the largest known among all the RNA viruses and it comprises up to 32kb of (+)ssRNA. It has 5’ and 3’ UTR, the polymerase gene and the structural protein genes. The sequential organization of the structural proteins is the same among all the coronaviruses: HE-S-E-M-N, where HE can be present or absent depending on the species. Interspersed the structural genes are several genes of unknown function and which number depends on the species. These genes can encode from one to three proteins, which translations most of the time depends on the preceding ORF acting as a ribosome binding site.5
It has been proposed that some of these proteins may play the role of antagonists of the immune responses as they are mostly not necessary for replication in vitro. When the virus makes its way through the cell membrane, it releases the viral ribonucleoprotein into the cytoplasm and the gene 1 is then translated. The product is two polyproteins (pp1a and pp1b) which are then cleaved into 15-16 proteins.5
The membrane associated replicase complex is formed by several proteins which includes the RdRp. One of the characteristics of the coronavirus is the presence of subgenomic mRNAs which are generated by a discontinuous process at the beginning of each gene in a sequence called the transcription regulatory sequence (TRS). When the polymerase reaches this site, the RNA synthesis stops and continues when it reaches the leader 5’ end of the genomic RNA, resulting in a negative sense copy of a subgenomic mRNA (sg mRNA). When the polymerase doesn’t stop at the TRSs, it generates a set of (-) sg mRNAs from which (+) sg mRNAs take place.5
The cultivability of MERS-CoV has permitted to have its complete genome sequenced. The MERS-CoV is 30119nt long and has at least 10 ORFs predicted. The 5’-UTR is 278nt long and the 3’-UTR 300. ORF1a and ORF1b are the longest ones, occupying ¾ of the genome and are located at its 5’-end. These ORFs are translated into polyproteins (pp1a and pp1b). 6 The four structural proteins are located downstream of ORF1b and interspersed them localize five accessory proteins that are unique to MERS-CoV (Figure 2)....