Molecular cloning of the Mason-Pfizer monkey virus genome: biological characterization of genome length clones and molecular comparisons to other retroviruses.

ABSTRACT

The molecular cloning of the DNA provirus of Mason-Pfizer monkey virus (M-PMV) is described. Fourteen independent clones of integrated M-PMV proviruses were isolated from a human embryo kidney cell line that had been previously derived from a single cell clone infected with M-PMV. Characterization of these clones for size of insert, restriction pattern of flanking DNA, and presence of repetitive DNA in the flanking sequences revealed that 10 of the isolates were identical while the four remaining clones were unique. Three independent clones of unintegrated M-PMV proviruses containing a single copy of the long terminal repeat (LTR) were cloned from acutely infected human embryo kidney cells, Transfection assays revealed that 13 of 14 integrated proviruses and 2 of 3 unintegrated proviruses were capable of producing infectious virus. One of the integrated provirus clones (clone 6A) produced consistently higher titers of virus than all of the other clones in all assays used and in two different cell lines, indicating that it contained a mutation that enhances virus replication. The virus recovered after transfection was shown to be capable of inducing cell fusion in nontransformed cell lines, confirming that this property is associated with M-PMV. One of the clones was hybridized under conditions of varying stringency, to molecular clones of type B, C, and D retroviruses. These studies revealed M-PMV to be most closely related to squirrel monkey retrovirus (D-type virus) and more distantly related to mouse mammary tumor virus (B-type virus). Hybridization was also detected with clones from the pol gene region of a family of human endogenous sequences. No homology was detected with Rous sarcoma virus or most mammalian C-type viruses tested. The exceptions were baboon endogenous virus and RD114 in which previously identified homology in the env gene was confirmed. These results suggest that the type D and type B viruses can be linked together in a group of viruses of similar ancestral origin analogous to that recently proposed for the human T-cell leukemia viruses and bovine leukemia virus.