The SARS-CoV nsp12 Polymerase Active Site Is Tuned for Large-Genome Replication.

Area of Science:

• Virology
• Molecular Biology
• Evolutionary Biology

Background:

• Positive-strand RNA viruses utilize RNA-dependent RNA polymerases (RdRPs) for genome replication, with conserved active-site structures.
• Coronaviruses (CoVs) possess large genomes and employ multi-protein replication-transcription complexes, including the core RdRP (nsp12) with essential subunits (nsp7, nsp8).
• Replicating large RNA genomes poses a challenge, requiring fast synthesis to evade host defenses, yet faster polymerases often exhibit lower fidelity.

Purpose of the Study:

• To investigate the mechanisms by which coronaviruses achieve rapid and accurate genome replication.
• To elucidate the roles of specific active-site residues in the coronavirus RdRP in modulating replication rate and fidelity.
• To understand the evolutionary adaptations of viral polymerases for accommodating large genomes.

Main Methods:

• Assembly kinetics of the CoV replication complex, including the use of a novel nsp8L7 heterodimer fusion protein.
• Rapid kinetics assays to measure RNA polymerase elongation rates.
• Biochemical analysis of specific active-site mutations (Ala547 and Ser759) in the RdRP.

Main Results:

• Preincubation of nsp12 with nsp8 accelerated complex assembly fivefold; a heterodimer fusion further optimized it.
• The core CoV replicase exhibited unusually fast elongation rates of up to 260 nucleotides/second.
• The active-site mutation Ala547 doubled replication rate at a fidelity cost, which was mitigated by the SDD (Ser759) mutation in the palm domain.

Conclusions:

• Mutations in polymerase motifs F and C, specifically Ala547 and the SDD sequence, are key to nidovirus evolution, tuning replication rate and fidelity for large genomes.
• Coronavirus RdRPs possess intrinsic mechanisms to achieve high replication speed while maintaining genome integrity, complementing known repair systems.
• These findings highlight the evolutionary adaptability of the core viral polymerase platform, enabling the transition from short-genome to long-genome viruses.