A Novel c-fgr Exon Utilized in Epstein-Barr Virus-Infected B Lymphocytes but Not in Normal Monocytes

Abstract

The fgr proto-oncogene encodes a nonreceptor protein-tyrosine kinase, designated p55^c-fgr. In this study, we have isolated human fgr cDNA molecules from normal monocyte mRNA templates. Nucleotide sequence analysis of the longest fgr cDNA revealed a 5′ untranslated region of 927 bp which included two Alu-like repeats as well as three translation stop codons immediately upstream of the initiator for p55^c-fgr synthesis. Within genomic DNA, these sequences were distributed over 13 kbp as three distinct 5′ untranslated exons. Previous studies have shown that Epstein-Barr virus (EBV) increases c-fgr mRNA levels in B lymphocytes. By comparing the nucleotide sequence reported for transcripts isolated from EBV-infected B lymphocytes with those of our monocyte cDNA as well as genomic DNA, we identified a novel untranslated exon utilized only in EBV-infected cells. The transcriptional initiation sites of fgr mRNA expressed in EBV-converted cells were mapped and shown to reside within a region identified as an intron for fgr mRNA that is expressed in normal myelomonocytic cells. Furthermore, the region of the fgr locus upstream of the novel exon displayed properties of a transcriptional promoter when transfected into heterologous cells. We conclude from all of these findings that activation of the fgr gene by EBV is achieved by mechanisms distinct from those normally regulating its programmed expression in myelomonocytic cells.