Expression of human foamy virus reverse transcriptase involves a spliced pol mRNA.
In human foamy virus (HFV) the reverse transcriptase is expressed independently of the Gag protein as a 127-kDa Pol precursor molecule. Evaluating the mechanism of Pol expression we identified a spliced mRNA which uses the main 5' splice donor and a splice acceptor site located in the gag gene. The significance of this spliced transcript for HFV Pol expression was studied by constructing a virus with a mutated splice acceptor site. This virus was unable to express detectable Pol proteins after transient transfection. Replication of the mutant was studied by a sensitive assay based on HFV transactivator-stimulated expression of an integrated lacZ gene under control of the HFV long terminal repeat. Whereas in the first 2 weeks after transfection the mutant replicated 3 to 5 order of magnitude less well than wild-type virus, extracellular titers obtained thereafter were similar to those of wild-type virus. This increase in replication competence was accompanied by a reversion of the mutated splice acceptor site. The results underlined the importance of the spliced pol transcript for HFV replication and pointed to a second mechanism of Pol expression. Indicator gene assays suggest that this other mechanism is likely to be a transactivator-dependent cryptic promoter in the gag gene which gives rise to Pol-encoding transcripts.
Jordan
I
I
Institut für Virologie und Immunobiologie, Würzburg, Germany.
Enssle
J
J
Güttler
E
E
Mauer
B
B
Rethwilm
A
A
D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D015870:Gene Expression; D015684:Gene Products, pol / Q000235:genetics*; D006801:Humans; D011498:Protein Precursors / Q000235:genetics; D012326:RNA Splicing; D012333:RNA, Messenger; D012194:RNA-Directed DNA Polymerase / Q000235:genetics*; D016092:Spumavirus / Q000201:enzymology* / Q000235:genetics
D015684:Gene Products, pol; D011498:Protein Precursors; D012333:RNA, Messenger; D012194:RNA-Directed DNA Polymerase