Frequent generation of nonrescuable reorganized Moloney murine sarcoma viral genomes

Nonproducer transformants infected with wild-type Moloney murine sarcoma virus were screened for the generation of variants with reorganized genomes. Seven of 20 lines contained such viral genomes, 5 of which were found to be nonrescuable. Blotting analysis indicated that viral RNA molecules transcribed from these variant genomes could not be encapsidated into virions. The nonrescuable genomes were molecularly cloned and all were found to have suffered deletions or deletions/inversions involving the 5' long terminal repeat as well as some adjacent sequences. Nucleotide sequence analysis suggested that the long terminal repeat or the tetranucleotides G-G-T-C and G-A-C-C (or both) were involved in the generation of these mutants. Transfection studies showed that the cloned DNAs of the 5 mutants transformed NIH/3T3 monolayers. Removal of the 3' long terminal repeat from the genomes that lacked the 5' long terminal repeat or carried it in an inverted orientation abolished or considerably reduced the transforming activity.