3.1. The 1ab polyprotein
The 1ab polyproteins of SARS-CoV-2, SARS-CoV and BatCoV RaTG13 are
composed of 7096, 7073, and 7095 amino acids, respectively (Table 1).
The amino acid sequence identity and similarity of the 1ab polyprotein
of the SARS-CoV-2 and the BatCoV RaTG13 were 98.5% and 99.1%,
respectively. The identity and similarity percentages of the 1ab
polyprotein of the SARS-CoV-2 and SARS-CoV were 86.2% and 92.9%.,
respectively. The results show that the SARS-CoV-2 is most probably
originated from Rhinolophus affinis bat rather than laboratory
modified SARS-CoV (Table.2).
After the production of the 1ab polyprotein, some endopeptidases in it
produce the 1a polyprotein and 16 nonstructural proteins [20]. The
cleavage products of the 1ab polyprotein carryout a wide range of
activities associated with the replication of the virus. The activities
include: binding and break down of ATP to produce ADP and phosphate,
different endopeptidases leading to formation of nonstructural proteins
(nonstructural proteins nsp3 and nsp5), production of ribose-
5-phosphate through exonuclease activity, synthesis of new nucleotides
by the methyltransferase, RNA polymerase and helicase for the viral
replication and removal of super twisting, and regulation of the
transcription through zinc finger proteins [20].