SARS coronavirus 2 /Covid-19 genome expression
Genome: single stranded RNA messenger 29.9kb long, encoding 13 ORFs. Coronavirus genomes have the longuest RNA virus genome known. Their RNA-dependent_RNA polymerase is also the only known to display proofreading function, maybe to stabilize this long RNA sequence. The proteins are expressed by two ways: primary translation of polyprotein that initiates the infection, and after some replication, subgenomic mRNA expression which produces all structural proteins.
Protein naming: Non structural proteins can be produced from the polyprotein or subgenomic mRNAs. In the first case those are numbered relative to the polyprotein from N to C terminal and called NSP: NSP 1->16, in the second case relative to the sgRNA number and called ORF: ORF3a->ORF9b. When two ORFs are expressed from the same sgRNA those are called a and b, example N(ORF9a) and ORF9b.
Polyprotein products expression
Subgenomic mRNA products
Subgenomic RNAs (sgRNAs) are created by discontinous transcription. During transcription of minus strand RNA, the polymerase have chances to pause on transcription-regulating sequences (TRS) and jump to leader TRS, thereby creating a major deletion. This creates a set of 9 RNAs that are subsequently replicated and transcribed. sgRNAS allows translation of all the structural proteins. The figure belows illustrate the discontinous transcription leading into 10 different RNAs . Only mRNA1 is encapsided and assembled in virions.