Viruses depend on the host cell’s translation apparatus and consequently, the outcome of infection is determined by the balance between a host’s ability to repress viral translation via innate immune responses, and viruses’ abilities to counteract them and usurp the translation apparatus. Coronavirus disease 2019 (COVID-19) is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), a beta coronavirus of the family Coronaviridae that also includes the clinically important SARS-CoV and MERS- CoV. During infection, coronaviruses (CoVs) utilize a dual strategy of suppressing translation and inducing degradation of cellular mRNAs while selectively enabling viral mRNAs to gain access to the cellular translation apparatus. This strategy is mediated by the viral non-structural protein Nsp1 that binds to 40S ribosomal subunits and induces a shutdown of host protein synthesis by two mechanisms: by direct stalling of translation of cellular mRNAs, and by inducing their endonucleolytic cleavage and subsequent degradation. 5’- untranslated regions of CoV genomic and all subgenomic mRNAs contain a common ~60-70 nucleotide-long element that includes the stem-loop SL1 that confers resistance of viral mRNAs to Nsp1-mediated translational suppression and endonucleolytic cleavage. These processes are critical for viral replication and pathogenesis and although they have emerged as potential targets for chemotherapeutic inhibitors that could have broad anti-coronaviral application, they remain poorly understood: the factor requirements and molecular details of initiation on genomic and subgenomic CoV mRNAs have never been determined, the mechanism of viral evasion of Nsp1-mediated translational shut-off is obscure, and the endonuclease that is responsible for Nsp1- induced cleavage of cellular mRNAs as well as the mechanism of its recruitment to ribosomal complexes are unknown. We propose to elucidate the mechanisms of these processes by recapitulating them in vitro using individual purified translational components and dissecting their individual stages using an array of biochemical techniques. In Aim 1, we will obtain a comprehensive overview of initiation on genomic and subgenomic SARV-CoV2 mRNAs by determining the complete set of required factors, characterizing the mechanisms by which they act in this process, and by identifying properties of these mRNAs that are responsible for unique aspects of the CoV initiation process. In Aim 2, we propose to characterize the influence of Nsp1 on all stages of initiation on cellular mRNAs and to investigate the mechanism of viral evasion of Nsp1-mediated translational shut-off. Aim 3 will focus on identification of the cellular endonuclease that mediates Nsp1- induced cleavage of host cell's mRNAs, characterization of the mechanism of its action, and identification of elements in viral mRNAs that confer resistance to endonucleolytic cleavage.