A tRNA gene potential to activate interferon signaling involves selective termination and is suppressible by La protein/SSB.

RNA polymerase III (Pol III) and some of its small non-coding RNAs have been co-opted by the immune system. We discovered a non-canonical activity for a human tRNA gene, TRT-TGT4-1, robust activation of interferon-stimulated gene (ISG) response after transfection into HEK293T cells, dependent on Pol III promoter-directed transcription and its flanking sequences. The activity requires the pattern recognition receptor RIG-I, which is activated by short RNAs with 5'-triphosphate (5'ppp). We characterized sequence-specific features of the TRT-TGT4-1 Pol III minimal T(4) terminator element which is uniquely required for and controls its ISG activity. Related, La protein, the Pol III transcript U(n)-3' terminus-binding protein, can promote and/or suppress the activity. Screening identified other tRNA genes with ISG activity whose subsequent analysis revealed the ISG-positive and ISG-negative ones differ most significantly in the 5'-dinucleotides preceding their terminators and T(n) length. Replacing the ISG-positive consensus 5'AC of TRT-TGT4-1 with an ISG-negative terminator 5'GG, markedly decreased termination and ISG activity. The data support a model in which Pol III terminators can channel transcripts and affect their potential for ISG activity or tRNA maturation. The findings reveal insight into other tRNA gene features involved in ISG signaling, and suggest some tRNA genes may express such potential.