Molecular Advances in Severe Acute Respiratory Syndrome-associated Coronavirus (SARS-CoV)

Chow, Ken Yan Ching; Hon, Chung Chau; Hui, Raymond Kin Hi; Wong, Raymond Tsz Yeung; Yip, Chi Wai; Zeng, Fanya; Leung, Frederick Chi Ching

The replicase gene of the SARS-CoV encodes for at least two proteins as a consequence of the proteolytic processing of the large polyprotein (ORF 1a and 1b; ref. 16 ). The translation of segment 1b of such polyprotein is interrupted by the −1 ribosomal frame shifting by a putative "slippery" sequence and a putative pseudoknot structure (16 ) . Two functional domains-papain-like cysteine proteinase (PL2 PRO ) and 3C-like cysteine proteinase (3CL PRO ), were identified experimentally and were responsible for the proteolytic processing of the polyprotein into 16 subunits (16 , 22 , 23 ) . A 375-a.a. SARS-CoV unique domain was identified upstream of the PL2 PRO domain, which is unparalleled in any other known coronaviruses (16 ) . In addition, seven more putative regions encoding RNA processing enzymes were identified, namely, RNA-dependent RNA polymerase (RDRP), RNA helicase (HEL) poly (U)specific endoribonuclease (XendoU), 30-to-50 exonuclease (ExoN), S-adenosylmethionine-dependent ribose 20-O-methyltransferase (20-O-MT), adenosine diphosphate-ribose 100-phosphatase (ADRP), and a cyclic phosphodiesterase (CPD; ref. 16 ). The translation of two polyproteins from ORF 1a and 1b starts the genome expression. The two proteinases, PCL2 PRO and 3CL PRO , are then coupled with the proteolytic processing of the two polyproteins into 16 units. PCL2 PRO is responsible for the N-proximal cleavage and 3CL PRO is responsible for the C-proximal cleavage. The helicase is then released. ATPase activity and DNA duplex-unwinding activity were demonstrated by purified helicase, indicating that the protein has RNA polymerase activity (16 , 24 ) .