The U3 region of Moloney murine leukemia virus contains position-independent cis-acting sequences involved in the nuclear export of full-length viral transcripts.

The distinguishing feature of self-inactivating (SIN) retroviral vectors is the deletion of the enhancer/promoter sequences in the U3 region of the 3' long terminal repeat. This design is used to overcome transcriptional interference and prevent downstream transcription from the 3' long terminal repeat. SIN vectors were derived from a number of different retroviruses. Studies of SIN vectors show that extensive U3 deletions in HIV-based vectors do not alter viral titers or the in vitro and in vivo properties of the vectors. However, deletion of the U3 sequences in γ- and α-retroviruses correlates with defects in 3' RNA processing and reduces viral titers by >10-fold. Here, we studied the steps in the retroviral life cycle that are affected by the deletion of sequences in the 3' U3 region of Moloney murine leukemia virus-derived retroviral vectors. The results show that the amounts of both full-length and internal RNA transcripts of U3-minus vectors are reduced in the nuclei of transfected cells, an effect that is probably due to a general defect in 3' RNA processing. Furthermore, full-length RNA transcripts were also defective in terms of nuclear export. This defect was complemented by transferring the U3 region to another position within the retroviral vector, indicating that the U3 region contains position-independent cis-acting sequences that are required for the transport of full-length viral transcripts. The results also suggest that the leader region of Moloney murine leukemia virus contains inhibitory/regulatory sequences, which prevent export and mediate nuclear retention of full-length viral RNA.