sidered to be background, was also observed. FLuc from all the tested dl VAR RNAs was expressed well in excess of dl DEMCV background levels, demonstrating that in the context of a bicistronic RNA the VAR 59UTRs are certainly capable of driving cap-independent translation in Xenopus laevis oocytes. Again the translational activity of the VAR 59UTRs was greater than that of the control pNL4.3 59UTR, while the IRES activities of the HXB2 and pNL4.3 59UTR were comparable. Findings also suggest that RNA splicing is not a requirement for the expression of the FLuc HIV-1 IRES reporter in Xenopus laevis oocytes. As an additional control dl DEMCV, dl HIV-1 IRES, and dl VAR DNA plasmids were microinjected into the cytoplasm of Xenopus laevis oocytes. In all cases neither RLuc nor FLuc activity could be detected, eliminating the possibility of de novo RNA synthesis from a DNA template. Together, these data provide substantial evidence supporting the presence of IRESes within the VAR 59UTRs examined. Furthermore, our observations are in 4 HIV-1 IRES RLuc and FLuc activities confirmed that the reporter proteins were not expressed for any of the dl vectors lacking the SV40 promoter even though DNA was present in cells. Detection of the full length bicistronic mRNA is not sufficient to fully discard the production of shorter aberrant RNA species. Nonetheless, the lack of luciferase activity when using the promoterless bicistronic RNAs strongly suggests that if shorter aberrant RNA species are formed their contribution to the overall RLuc and FLuc activity is marginal. Results confirm that the full length bicistronic mRNA accounts for most, if not for all, the expression of the luciferase reporters the bicistronic constructs. Careful analysis of the data presented in figure 4C shows low levels of FLuc activity in the dl VAR 12 lacking the SV40 promoter. Thus, in this particular construct dl VAR 12, we cannot definitively exclude the presence of a cryptic promoter. Further experiments are required to absolutely discard this possibility. Nonetheless, and based on the same analysis we exclude the possibility PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189787 that FLuc activity from the dl VAR 1, dl VAR 2, dl VAR 3, and dl VAR 4 constructs PD173074 supplier observed in HeLa cells is due to the protein production from an additional transcript encoding FLuc that was generated via a cryptic promoter. Chemical and Enzymatic Probing of the Variant HIV-1 59UTRs agreement with previous studies, and imply that additional viral proteins are not required to obtain a basal level of HIV-1 IRES activity in Xenopus laevis oocytes. Expression of FLuc in the Bicistronic mRNAs is not Due to Cryptic Promoter Activity The firefly luciferase reporter gene has recently been shown to exhibit a cryptic promoter activity that is detectable both in yeast and in mammalian cells. Even though the identified promoter lies within the FLuc coding sequence and is expected to generate shorter mRNAs that do not code for the functional luciferase enzyme, we were interested in establishing if possible aberrant transcripts may be influencing the results of the experiments described above. For this, we removed the SV40 promoter from the bicistronic constructs. In this setting, expression of the downstream transgene would imply that the DNA coding the bicistronic mRNA possesses a 
cryptic promoter activity that gives rise to at least an RNA species that engenders a functional luciferase enzyme. If so, this aberrant gene expression would be clouding our interpret