Like togaviruses and flaviviruses, coronaviruses have the following properties:
- + strand ssRNA genomes
- Use insects as secondary hosts
- Wide range and severity of diseases
Coronaviruses have a helical nucleocapsid. They also have an enormous genome (4-5 times larger than that of picornaviruses). While togaviruses use only two mRNAs to synthesize its proteins, coronaviruses use 7. Each coronavirus mRNA encodes for a different protins. Each coronavirus mRNA has the same 3' sequence (including polyA), but each one begins at a different point on the genomic strand. Only the most 5' gene is translated from each subgenomic RNA. We will analyze the replication cycle of Mouse Hepatitis Virus (MHV) for further detail:
|Step 1||The virus infects and synthesizes a minus strand.|
The minus strand acts as a template for all the subgenomic RNAs. Each one is used to produce a different protein, which are then trafficked through host cells protein processing organelles (ER, Golgi) before being assembled with the nucleocapsid to make new virions (similar way to togavirus). Even though each subgenomic RNA begins at a different positiion relative to genomic RNA, sequencing of subgenomic RNAs revealed that each one had exactly the same small leader sequence at 5' end (same as the 5' end of genomic RNA). There are 3 models to explain this, which involve repeated sequences between coding regions, called interegenic sequences (IS), and these are short sequences which are homologous to the 3' end of the leader sequence:
Like togaviruses, coronaviruses are mainly distributed in the Americas, Africa, and Asia. They prefer topical, hot, and humid climates. Most of the viruses (especially Î±) use insects/mosquitos/ticks as 2° hosts. They use other animals as reservoirs to maintain virus in nature when 1Âº is not available.