A promoterless retroviral vector indicates that there are sequences in U3 required for 3' RNA processing.

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Abstract
We constructed a retrovirus vector in which all viral transcriptional signals are deleted during provirus formation. This vector would be safer and more useful in gene-transfer experiments. Construction of this vector involved the deletion of all of the U3 sequence except for 10 base pairs, required for integration, from the right-side long terminal repeat. We found that this deletion resulted in an inability to propagate virus efficiently. When we inserted sequences containing all of the signals for polyadenylation of simian virus 40 late mRNA at the end of our vector, we were able to propagate virus efficiently. This result indicates that there are sequences in U3 upstream from AAUAAA that are essential for 3'' processing of viral RNA.