Figure 4.

Summary of MLV U3 end mutants used in this study. Deletions shown in the boxes are at the U3 ends of both LTRs. ▴ indicates the position of the deletion, and • indicates the strong enhancement observed at ≈20 bp from the U3 end. Viral replication was determined by measuring the reverse transcriptase activity in culture supernatants. Integration activity was assayed with preintegration complexes in the corresponding cytoplasmic extracts. MM-PCR was used to assay for the presence of the characteristic enhancements (see Fig. 5).