The envelope () gene of human T cell leukaemia virus type I (HTLV-I) was inserted into an expression vector, referred to as phMTenv, under the transcriptional control of the human metallothionein IIa gene promoter (hMT-IIa). When this vector was transiently transfected in HeLa cells treated with hMT-IIa inducers, formation of multinucleated cells was observed, indicating the expression of functional surface and transmembrane glycoproteins. Of several HeLa cell clones transfected with phMTenv together with a plasmid carrying the neomycin resistance gene and isolated after selection in G418-containing medium, mRNA was detected in only two, in the presence of hMT-IIa inducers. Viral glycoproteins were found to be weakly expressed, as detected in immunoprecipitation assays of I-surface-labelled cells. These -transfected HeLa cell clones, although unable to form syncytia when cocultivated with untransfected control HeLa cells, retained the capacity to fuse with HTLV-I-producing C91PL T cells. However, a significant decrease in their fusogenic ability was observed, after treatment with hMT-IIa inducers. Under identical experimental conditions, control HeLa cell clones stably transformed with the same plasmid, but lacking the gene, were still able to fuse with C91PL cells. These observations suggest that a post-transcriptional step in HTLV-I expression is impaired, probably leading to the establishment of superinfection interference.


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