Characterization of Rous sarcoma virus sequences essential for viral gene expression.

Using the Escherichia coli lacZ gene product beta-galactosidase as an indicator of gene expression, we analyzed sequences that are required for expression of the Rous sarcoma virus (RSV) genome in avian cells. The RSV long terminal repeat (LTR) and leader region were sufficient to direct the synthesis of high levels of enzymatically active gag-lacZ fusion proteins. A portion of U3 greater than 140 nucleotides upstream from the cap site was essential for gene expression. This element functioned in either orientation, but its activity was attenuated when it was relocated further away from the cap site. The insertion of exogenous LTRs 3' of lacZ augmented the expression of that gene by increasing the level of stable gag-lacZ transcripts. Furthermore, 3' LTRs could partially compensate for certain defects within the 5' LTR. Insertion of various fragmentary LTRs allowed the identification of at least three synergistically acting domains within the 3' LTR that influence gene expression. Interestingly, the gag-lacZ expression was only stimulated by a 3' LTR when the exogenous 3'-untranslated region was adjacent. Our results imply that the two LTRs of a provirus interact in a complex manner to promote high levels of stable transcripts. It was also found that gag-lacZ expression was independent of viral gene products, suggesting that trans-activation is not a key mechanism regulating RSV expression in avian cells.