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CoronavirusComments

Coronavirus

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 1The virus infects and synthesizes a minus strand.
Step 2

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:

Exp'tion 1Leader-primed transcription has 2 key points: the genome encodes a minus RNA as the template for all subgenomic RNAs, and the IS serves as a promoter. A leader sequence is formed; RNA synthesis stops; leader sequence dissociates and reassociates further down by binding to the IS; leader sequence is the primer for the rest fo the chain; this is supported by the finding that there are free short elader sequences in infected cells.
Exp'tion 2Modified leader-primed transcription has the same key points as leader-primed transcription. Rather than the polymerase and leader sequence jumping form the leader to the IS, these 2 reginos are brought together and intervening RNA is looped out. The sequences are united by proteins which to each sequence, and then bind to each other. The polymerase can then synthesize RNA across the 2 regions to make a continuous strand. This is supported becase synthesis of full-lengthed plus strands is inhibited in favor of subgenomic fragments (this would be because looping prevents minus RNA from serving as a template for synthesis across its entire length).
Exp'tion 3Discontinuous transcription has 2 key points: IS is the terminator of transcription, and there is discontinuous transcription. The minus strand is made as short segments, which are templates for each plus mRNA; the mRNAs are short because the minus strands themselves are short RNAs

Like togaviruses, coronaviruses are mainly distributed in the Americas, Africa, and Asia. They prefer topical, hot, and humid climates. Most of the viruses (especially α) use insects/mosquitos/ticks as 2° hosts. They use other animals as reservoirs to maintain virus in nature when 1º is not available.