Previously, we described and characterized yeast non-chromosomal determinant [NSI+], possessing prion properties. This determinant causes a decrease in translation termination fidelity, which is phenotypically detectable as the nonsense suppression in the strains with decreased functional activity of eRF3 release factor. As a result of geneticscreen, we demonstrated that an increase in the expression of SUP45 encoding the eRF1 release factor (Sup45), masks, but does not eliminate nonsense suppression in the [NSI+] strains. In the present study, we first demonstrated the direct cause for the nonsense suppression in [NSI+] strains. We demonstrated that [NSI+] decreases the relative amounts of SUP45 mRNA that causes a decrease in the amounts of Sup45 protein that is detectable in the stationary growth phase. The data obtained suggest the structural protein of [NSI+] seems to be either a transcription factor or participates in the regulation of cellular mRNA stability.