Selective packaging of host tRNA's by murine leukemia virus particles does not require genomic RNA

The 4S RNA contained in RNA tumor virus particles consists of a selected population of host tRNA's. However, the mechanism by which virions select host tRNA's has not been elucidated. We have considered a model which specifies that 35S genomic RNA determines which tRNA's are to be encapsidated as well as the relative amounts of these tRNA's within the virion. The model was tested by comparing the free 4S RNA composition of normal murine leukemia virus (MuLV) particles and noninfectious virions from actinomycin D (ActD)-treated cells, which are deficient in genomic RNA (ActD virions). Viral 4S RNA was analyzed by two-dimensional polyacrylamide gel electrophoresis. Surprisingly, the patterns obtained for control and ActD 4S RNA were identical to each other and were clearly distinct from the cell 4S RNA pattern. The viral patterns had three prominent areas of radioactivity. One of the spots was identified on the basis of its oligonucleotide fingerprint as tRNA (Pro), the primer for MuLV RNA-directed DNA synthesis. These results were obtained with two different MuLV strains, AKR and Moloney, each grown in SC-1 cells. The demonstration that ActD virions contain primer tRNA and in general exhibit the characteristic MuLV tRNA pattern rather than the complete representation of cell 4S RNA leads to the conclusion that genomic RNA is not the major determinant in selective packaging of host tRNA's. A possible role for one or more viral proteins, including reverse transcriptase, is suggested.