ANALYSIS OF THE ROLE OF THE BEL AND BET OPEN READING FRAMES OF HUMAN FOAMY VIRUS BY USING A NEW QUANTITATIVE ASSAY

We have constructed a BHK-21-derived indicator cell line containing a single integrated copy of the beta-galactosidase (beta-Gal) gene under control of the human foamy virus (HFV) long terminal repeat promoter (from -533 to +20). These foamy virus-activated beta-Gal expression (F A-B) cells can be used in a quantitative assay to measure the infectio us titer of HFV. Our results show that the FAB assay is 50 times more sensitive than determination of the virus titer by the end-point dilut ion method. Using the FAB assay, we have found that HFV can productive ly replicate in several erythroblastoid cell lines as well as in the J urkat T-cell line. We have also examined the roles of bel2, bet, and b el3 in viral replication by constructing proviral HFV clones in which the reading frame of Bel2, Bet, or Bel3 is disrupted by placement of t ranslation stop codons. Analysis of these mutants reveals that while t he bel3 gene is not required for viral replication in vitro, mutations in the bel2 or bet gene decrease cell-free viral transmission approxi mately 10-fold.