rp contrast to the above prediction, TMG-capping of viral mRNAs increases the expression of HIV-1 structural proteins. The mechanism by which these TMG-capped viral mRNAs recruit the translational machinery remains unevaluated, yet it is tempting to speculate that translation initiation from the TMGHIV-1 RNAs is 
mostly IRES-dependent. This possibility would partially justify why the HIV-1 full length mRNA requires an IRES. An additional set of reports also advocate in favor of the requirement of a cap-independent initiation mechanism for the HIV-1 mRNA. IRES-mediated translation initiation has been proposed to allow the viral mRNA to bypass the constraints of global cellular translation repression that normally targets cap-dependent translation initiation. In the case of the HIV-1 mRNA, IRES-mediated initiation would support viral protein synthesis during the G2/M phase of the cell cycle and during osmotic stress. Additionally, capindependent translation initiation would ensure synthesis of HIV-1 structural proteins during the late stages of the replication cycle, when the eukaryotic initiation factor eIF4G and the poly binding protein, both required for cap-dependent translation initiation, are targeted by the HIV-1 RGFA-8 site protease. In consequence, harboring an IRES would allow the HIV-1 mRNA to overcome translational constrains that specifically target cap-dependent translation initiation imposed in part by the viral replication cycle itself. Materials and Methods Viral RNA Purification Surplus total RNA, extracted using the High Pure Viral Nucleic Acid kit from the serum of HIV-1 infected patients, normally discarded upon viral load determination, was randomly pooled and used in this study as the source of viral RNA. Pooled RNA samples were provided by the Laboratorio de Infectologia, Facultad de Medicina, Pontificia Universidad Catolica de Chile. Plasmid Construction The dl DEMCV and the dl HIV-1 IRES plasmids were previously described. For the generation of the bicistronic vectors dl VAR, the 59UTR of natural variants were recovered from the randomly pooled viral RNA extracts by RT-PCR using the SuperScriptTMIII one step RT-PCR system with platinumH Taq DNA polymerase using the primers HIV1-F and HIV1-R. All new sequences identified in this study can be found in GenBank. The dl VAR vectors were generated as previously described, in brief the amplicon was digested with EcoRI and NcoI and ligated, using a triple ligation strategy, with the 5248 bp-EcoRI/XbaI and 1656 bp -NcoI/XbaI fragments of the previously digested dl HIV-1 IRES. To generate plasmids without the SV40 mammalian promoter, the bicistronic vectors were digested with StuI and MluI, treated with the E. coli DNA Polymerase I Klenow fragment to generate blunt ends, and ligated using T4 DNA ligase. All plasmids PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22189542 were confirmed by sequencing. Cell Culture HeLa cells, similar to those used in previous studies, kindly provided by Dr. Nahum Sonenberg, were cultured in Dulbecco’s modified Eagle’s medium with 50 U/mL of penicillin-streptomycin and 10% fetal bovine serum at 37uC in a 5% CO2 atmosphere. DNA Transfection Cells were seeded at 105cell/well in 12-well plates. DNA transfection was performed at 60% confluency by the JetPei system according to the manufacturer’s protocol. For the hnRNPA1 over expression HIV-1 IRES experiments, the well characterized hnRNPA1 expression plasmid , kindly provided by Dr. Andrew Mouland and Benoit Chabot or the empty vector DNA were transfected in cel