Open reading frame 9a (ORF9a) of SARS-CoV-1 mRNA9 has a length of 1269 nt and encodes the 422 aa pre-nucleoprotein, the most abundant viral protein expressed during infection. Nucleoprotein is translated by cytosolic free ribosomes and most of it stays in the cytosol where it soon colocalizes with nsp3 and viral genomic RNA. It is involved in replication and transcription of the viral genome, but it is also a structural component of the virion (Thiel et al, 2003; Li et al, 2005; Stertz et al, 2007; Fung & Liu, 2019).
By mass spectrometric analysis, it was shown that methionine residues of the N protein are sulfur oxygenated (Krokhin et al. 2003), but the physiological relevance of this modification is unknown. The serine at position 2 of the N protein may be acetylated (Arnesen et al. 2005).