Reverse transcription is a key event in the replication cycle of human immunodeficiency virus type 1 (HIV-1) that is initiated after the virus enters the cell. Events subsequent to reverse transcription include nuclear transport of the preintegration complex (PIC), targeting PICs to the sites of integration, proviral integration, and finally the repair of the termini by mechanisms not yet fully understood. Reverse transcription is initiated in the cytoplasm, and the viral cDNA is synthesized in the cytoplasmic compartment. However, it is unclear if reverse transcription is completed prior to proviral integration, as proviral DNA containing discontinuities in plus-strand DNA has been shown to efficiently integrate in vitro. Indeed, reverse transcriptase (RT) has been detected in PICs and intracellular reverse-transcription complexes isolated from the nuclei of HIV-1-infected cells (5, 11). Thus, it is possible that RT is required for polymerization subsequent to nuclear transport, including the completion of plus-strand DNA synthesis and for repair of the single-strand gaps that remain after integration. In certain retrotransposons, exemplified by R2Bm, reverse transcription begins after endonucleolytic cleavage of the genomic DNA by integrase (IN). The RNA is copied using the 3Ј-OH terminus generated by cleavage of the genomic DNA (24), which would necessitate the retention of RT in the nuclear PICs.Furthermore, in avian leukosis virus (14, 30) and in human T-lymphotropic leukemia virus type 1 (33), RT and IN are parts of a single polypeptide forming the ␣ subunit of the heterodimeric ␣ complex. In Rous sarcoma virus, where it has been well studied, the ␣ complex retains both RT and IN activities. In addition, the recent demonstration that the ␣ complex localizes to the nucleus suggests that the RT and IN proteins are likely present together in the nuclear PICs (37). In contrast, the RT and IN proteins of murine retroviruses and of HIV are fully separated by proteolytic cleavage during virion maturation (27). The observation that RT is present in the nuclear PICs (5) in addition to within reverse-transcription complexes (11) suggests that RT may be retained via proteinprotein or protein-nucleic acid interactions with other viral components.Previous studies led to the observation that the RT and IN proteins specifically interact with each other and that this interaction is not mediated by nucleic acid bridging (38). Here, we demonstrate that both monomeric and heterodimeric forms of RT can interact with IN, and we map the domains of interaction on both protein partners. Attempts to assess the effect of IN on RT function showed that RT function was unaffected by IN. In contrast, in the presence of RT, the IN-mediated joining reaction was stimulated significantly, while the 3Ј-end processing was unaffected. These results suggest functional interaction of the two proteins.