Regulation of the double-stranded RNA-dependent protein kinase PKR by RNAs encoded by a repeated sequence in the Epstein-Barr virus genome

During the initial infection of B lymphocytes by Epstein-Barr virus (EBV) only a few viral genes are expressed, six of which encode the EBV nuclear antigens, EBNAs 1-6, The majority of EBNA mRNAs share common 5'-ends containing a variable number of two alternating and repeated exons transcribed from the BamHI W major internal repeats of the viral DNA, These sequences can also exist as independent small RNA species in some EBV-infected cell types, We present evidence that transcripts from these W repeat regions can exert a trans-acting effect on protein synthesis, through their ability to activate the dsRNA-dependent protein kinase PKR, UV cross-linking and filter binding assays have demonstrated that the W transcripts bind specifically to PKR and can compete with another EBV-encoded small RNA, EBER-1, which was shown previously to bind this kinase, In the reticulocyte lysate system the W RNAs shut off protein synthesis through an ability to activate PKR, In contrast to EBER-1, the W RNAs are unable to block the dsRNA-dependent activation of PKR, Using a purified preparation of the protein kinase we have shown that the W transcripts directly activate PKR in vitro, The results suggest that EBV has the ability both to activate and to inhibit PKR through the actions of different products of viral transcription.