ABSTRACT
SARS-CoV-2 coronavirus is responsible for Covid-19 pandemic. In the early phase of infection, the single-strand positive RNA genome is translated into non-structural proteins (NSP). One of the first proteins produced during viral infection, NSP1, binds to the host ribosome and blocks the mRNA entry channel. This triggers translation inhibition of cellular translation. In spite of the presence of NSP1 on the ribosome, viral translation proceeds however. The molecular mechanism of the so-called viral evasion to NSP1 inhibition remains elusive. Here, we confirm that viral translation is maintained in the presence of NSP1. The evasion to NSP1-inhibition is mediated by the cis-acting RNA hairpin SL1 in the 5UTR of SARS-CoV-2. NSP1-evasion can be transferred on a reporter transcript by SL1 transplantation. The apical part of SL1 is only required for viral translation. We show that NSP1 remains bound on the ribosome during viral translation. We suggest that the interaction between NSP1 and SL1 frees the mRNA accommodation channel while maintaining NSP1 bound to the ribosome. Thus, NSP1 acts as a ribosome gatekeeper, shutting down host translation or fostering SARS-CoV-2 translation depending on the presence of the SL1 5UTR hairpin. SL1 is also present and necessary for translation of sub-genomic RNAs in the late phase of the infectious program. Consequently, therapeutic strategies targeting SL1 should affect viral translation at early and late stages of infection. Therefore, SL1 might be seen as a genuine Achille heel of the virus.
ABSTRACT
A synthetic gene encoding Escherichia coli tRNA(Arg)(2) was inserted in a plasmid under the control of an isopropyl-beta, D-thiogalactopyranoside (IPTG)-inducible promotor, pTrc99B. In E.coli MT102 transformed by the above plasmid containing the target gene. TRNA(Arg)(2) was overproduced up to 30 fold of that of the host. In the transformant the quantity contained tRNA(Arg) increased 10 times and was 70% of the total tRNA. The tRNA(Arg)(2) was purified to 88% homogeneity by passing through a DEAE-Sephacel column, and then was purified by benzyl-DEAE cellulose column chromatography to a purity of 99% with an arginylation activity of 1 600 pmole/A(260) unit. Eighteen milligrams of tRNA(Arg)(2) could be obtained from 40 mg total tRNA which was obtained from four liters of overnight culture, and the yield of the purification was 62%. The accurate kinetic constants of aminoacylation of tRNA(Arg)(2) catalyzed by arginyl-tRNA synthetase were comparable with that of tRNA(Arg) from Sigma.
