Sequence arrangement and biological activity of cloned feline leukemia virus proviruses from a virus-productive human cell line.

We examined 14 different feline leukemia virus proviruses from the productively infected human cell line RD(FeLV)-2 after cloning in the modified lambda vector Charon 4A. Each isolate was characterized by restriction digestion and Southern blot analysis. The DNA of each isolate was tested for competence to express virus after uptake by sensitive animal cells (transfection). All but one isolate contained an apparently complete provirus, but only four were infectious. Seven isolates (four noninfectious, three infectious) were studied by heteroduplexing followed by electron microscopy or by S1 nuclease treatment and gel electrophoresis. No regions of nonhomology between proviruses were detected by either criterion, and in no case did we observe homology between flanking sequences. Random shearing or removal of flanking sequences by S1 nuclease had no effect on the status of infectivity of the clones. Thus, we were unable to find molecular differences between infectious and noninfectious proviruses. Our data are consistent with either of the following hypotheses: (i) that there is a short host sequence which is essential as a promoter for virus expression; or (ii) that lack of infectivity is due to small mutations within the proviral genome.