RNA Dependent RNA Polymerases of Severe Acute Respiratory Syndrome-Related Coronaviruses- An Insight into their Active Sites and Mechanism of Action

Aim: To analyze the RNA-dependent RNA polymerases (RdRps) of Severe Acute Respiratory Syndrome (SARS)-related coronaviruses (CoVs) to find out the conserved motifs, metal binding sites and catalytic amino acids and propose a plausible mechanism of action for these enzymes, using SARS-CoV-2 RdRp as a model enzyme. Study Design: Bioinformatics, Biochemical, Site-directed mutagenesis (SDM), X-ray crystallographic and cryo-Electron microscopic (cryo-EM) data were analyzed. Methodology: Bioinformatics, Biochemical, Site-directed mutagenesis, X-ray crystallographic and cryo-EM data of these enzymes from RNA viral pathogens were analyzed. The advanced version of Clustal Omega was used for protein sequence analysis of the RdRps. Results: Multiple sequence alignment (MSA) of RdRps from different SARS-related CoVs show a large number of highly conserved motifs among them. Though the RdRp from the Middle Eastern Respiratory syndrome (MERS)-CoV differed in many conserved regions yet the active site regions are completely conserved. Possible catalytic regions consist of an absolutely conserved amino acid K, as in single subunit (SSU) RNA polymerases and most of the DNA dependent DNA polymerases (DdDps). The invariant ‘gatekeeper/DNA template binding’ YG pair that was reported in all SSU DNA dependent RNA polymerases (DdRps), prokaryotic multi-subunit (MSU) DdRps and DdDps is also highly conserved in the RdRps of SARS-CoVs. The universal metal binding motif –GDD- and an additional motif–SDD- are also found in all SARS-CoV RdRps. In stark contrast, the (–) strand RNA viral pathogens like Ebola, rabies, etc. use –GDN- rather than –GDD- for catalytic metal binding. An invariant YA pair (instead of an YG pair) is found in the primases of the SARS-CoVs. The SARS-CoVs RdRps and primases exhibit very similar active site and catalytic regions with almost same distance conservations between the template binding YG/YA pair and the catalytic K. In SARS-CoV RdRps an invariant R is placed at -5 which is shown to play a role in nucleoside triphosphate (NTP) selection and is in close agreement with SSU DdRps (viral family) and DdDps. In primases no such invariant R/K/H is found very close to the catalytic K in the downstream region, as found in RdRp and Nidovirus RdRp-Associated Nucleotidyltransferase (NiRAN) domains. An invariant YA pair is placed in the NiRAN domain instead of an YG pair, and an invariant H is placed at -5 position. Moreover, the Zn binding motif with the completely conserved Cs and a few DxD/DxxD type metal binding motifs are found in the RdRps and NiRAN domain. However, the primases contained only the DXD type metal binding motifs. Conclusions: The SARS and SARS-related CoV RdRps are very similar as large peptide regions are highly conserved among them. The closer identity between the RdRps of palm civet-CoV and SARS-CoV (CoV-1) suggest their possible link as found between their spike proteins also. The invariant YG and KL pairs may play a role in template binding and catalysis in SARS-CoV RdRps as reported in DdDps. An additional invariant –YAN- motif found in SARS-CoV RdRps may play a crucial role in nucleotide discrimination.