The R region found in the human foamy virus long terminal repeat is critical for both Gag and Pol protein expression

It has been suggested that sequences located within the 5' noncoding region of human foamy virus (HFV) are critical for expression of the viral Gag an d Pol structural proteins. Here, we identify a discrete similar to 151-nucl eotide sequence, located within the R region of the HFV long terminal repea t, that activates HFV Gag and Pol expression when present in the 5' noncodi ng region but that is inactive when inverted or when placed in the 3' nonco ding region. Sequences that are critical for the expression of both Gag and Pol include not only the 5' splice site positioned at +51 in the R region, which is used to generate the spliced pol mRNA but also intronic R sequenc es located well 3' to this splice site. Analysis of total cellular gag and pol mRNA expression demonstrates that deletion of the R region has little e ffect on gag mRNA levels but that R deletions that would be predicted to le ave the gal 5' splice site intact nevertheless inhibit the production of th e spliced pol mRNA. Gag expression can be largely rescued by the introducti on of an intron into the 5' noncoding sequence in place of the R region but not by an intron or any one of several distinct retroviral nuclear RNA exp ort sequences inserted into the mRNA 3' noncoding sequence. Neither the R e lement nor the introduced 5' intron markedly affects the cytoplasmic level of HFV gag mRNA, The poor translational utilization of these cytoplasmic mR NAs when the R region is not present in cis also extended to a cat indicato r gene linked to an internal ribosome entry site introduced into the 3' non coding region. Together these data imply that the HFV R region acts in the nucleus to modify the cytoplasmic fate of target HFV mRNA, The close simila rity between the role of the HFV R region revealed in this study and previo us data (M. Butsch, S. Hull, Y.Wang, T. M. Roberts, and K. Boris-Lawrie, J. Virol. 73:4847-4855, 1999) demonstrating a critical role for the R region in activating gene expression in the unrelated retrovirus spleen necrosis v irus suggests that several distinct retrovirus families may utilize a commo n yet novel mechanism for the posttranscriptional activation of viral struc tural protein expression.