Intron Retention May Regulate Expression of Epstein-Barr Virus Nuclear Antigen 3 Family Genes

The nuclear antigen 3 family genes (EBNA-3, EBNA-4, and EBNA-6) of Epstein-Barr virus (EBV) are important for EBV-induced immortalization and survival of B lymphocytes. However, little is known about how the expression of these genes is regulated. Each of the EBNA-3, EBNA-4, and EBNA-6 genes consists of two exons separated by a small intron. Reverse transcriptase PCR assays revealed that the vast majority of the EBNA-3, EBNA-4, and EBNA-6 mRNA, expressed in transfected and EBV-infected B cells, retained intron sequences. Northern blot and S1 protection assays confirmed that most of the EBNA-3 mRNA contained intron. Examination of deletion mutants of EBNA-3 indicated that the EBNA-3 protein was not necessary for intron retention and that there was no splicing silencing element encoded in the EBNA-3 mRNA. Cell fractionation and RNA gradient analysis revealed that the unspliced EBNA 3 family mRNAs were transported into the cytoplasm and associated with the polysomes. However, Western blot analysis of FLAG-epitope tagged EBNA-3 gave no indication of the presence of splice variant protein forms of EBNA-3. In contrast, transiently transfected cells expressing EBNA-3 revealed a sixfold increase in EBNA-3 protein expression from the genomic EBNA-3 gene compared to EBNA-3 cDNA. These data show that the intronic sequences can influence EBNA-3 protein expression and suggest that intron retention may provide a means for the fine-tuning of expression of the individual EBNA 3 family genes.