Retroviruses act as insertional mutagens and can also capture cellular sequences through a mechanism which initially requires the generation of RNA transcripts which fail to cleave and polyadenylate correctly. The correct termination of retroviral transcripts at the 3' LTR R/U5 junction is primarily dependent on the canonical AAUAAA polyadenylation signal, so we have analyzed the effect of mutating the polyadenylation signal sequences on the properties of a selectable murine retroviral vector. Mutation of consensus polyadenylation signal sequences in the 5' and/or 3' proviral LTRs demonstrated that a UA to GG change generated larger sized virus-specific RNA, consistent with loss of normal polyadenylation. Cell clones infected with viruses generated by proviral constructs containing this mutation in the 5' LTR express either normal-length or elongated viral RNA. Fused transcripts contained the mutant polyadenylation signal, while sequence analysis was consistent with the hypothesis that premature 5' to 3' primer strand transfer was responsible for the high frequency (80%) of wild-type polyadenylation. Cells infected by viruses from constructs mutated in both 5' and 3' proviral LTRs expressed poly(A)+ viral RNA between 0.3 and 3 kb larger than normal virus in 100% of infected clones, and sequence analysis of clones derived from either infected rodent or human cells confirmed that these transcripts contained both viral and adjacent cellular sequences. While mutant virus exhibits no increased ability to alter cell phenotypes, the read-through transcripts contain both unique and repetitive cell-derived sequences and can easily be recovered using PCR techniques, suggesting that these viruses may serve as effective tools for rapidly cloning cellular sequences and generating random genomic markers for gene mapping.
Copyright 1998 Academic Press.