Cellular DNA. The highest stimulatory effect of Rev could be observed on encapsidation of the RRE-containing Msd1-sa5 transcript encoded by VHenv (figure 4, blue squares). Encapsidation of the transcript SD1-SA5 expressed from 34540-22-2 web VHgenomic identical in sequence to Msd1-sa5 was also increased by Rev but to a lesser extent (figure 4, blue squares). Competition between unspliced and spliced 1326631 HIV transcripts was previously identified to diminish packaging of spliced RNAs [8,9]. Therefore, a possible explanation for the get 58-49-1 efficient Rev-mediated packaging of Msd1-sa5 is the lack of such a competition between the different transcripts of VHenv all of which do not contain the full-length encapsidation signal (figure 1 and figure 4). In contrast to the situation after transfection of VHenv, VHgenomic generates two transcripts, the unspliced and the singly-spliced SD1-SA5 RNA, which are exported to the cytoplasm by Rev. Since the unspliced transcript contains the full-length encapsidation signal that is truncated after splicing in the singly-spliced transcript, the presence of the unspliced RNA could limit encapsidation of the singly-spliced SD1-SA5 RNA. Encapsidation efficiencies of lentiviral vector transcripts lacking the RRE did not vary significantly with or without Rev (figure 4, red diamonds). A 2-fold increase was observed in the presence of Rev for these RNAs. It is possible that this small stimulatory trend is mediated by binding of Rev to the first RNA stem loop in the encapsidation signal present in all viral transcripts [28]. Surprisingly, encapsidation of the fully-spliced lentiviral vector transcriptswithout RRE was highly efficient in our experiments (figure 4), while fully-spliced transcripts of wild type HIV are poorly packaged [9]. Furthermore, the amount of virion-associated genomic RNA in comparison to spliced RNAs is 20 to 40-fold higher in wild type HIV particles [10,29,30]. In our experiments this effect was smaller ranging from 5-fold for fully-spliced to 17fold for singly-spliced RNAs (figure 3B). These facts demonstrate that the conditions in our experiments differed in some aspects from the wild type situation. A possible reason for these discrepancies could be that not all cells are cotransfected with every plasmid used. Cells transfected only with the Revindependent gag/gagpol expression plasmid together with the lentiviral vector but not with the rev expression plasmid could lead to particles containing mainly fully-spliced RNAs, because packaging of Rev-dependent singly-spliced and unspliced RNAs would be impaired. In addition, high intracellular RNA levels obtained after transient transfection of HEK293T cells with the CMV promoter driven lentiviral vectors in the presence of Tat are believed to reduce the specificity of the encapsidation process [5]. Furthermore, utilization of the Rev and Tat-independent gag/ gagpol expression plasmid could lead to increased packaging of fully-spliced RNA. In the HIV replication cycle the viral proteins Tat and Rev are expressed from multiply-spliced transcripts and have to accumulate to a certain threshold level to allow subsequent expression of genes from partially and unspliced RNAs [5,31?4]. Additionally, the viral protein Gag/GagPol is produced from the unspliced transcript. These facts guarantee a spatial and temporal regulated appearance of Gag and genomic RNA at the same site in the cytoplasm late in the replication cycle. Most likely this concerted gene expression pattern al.Cellular DNA. The highest stimulatory effect of Rev could be observed on encapsidation of the RRE-containing Msd1-sa5 transcript encoded by VHenv (figure 4, blue squares). Encapsidation of the transcript SD1-SA5 expressed from VHgenomic identical in sequence to Msd1-sa5 was also increased by Rev but to a lesser extent (figure 4, blue squares). Competition between unspliced and spliced 1326631 HIV transcripts was previously identified to diminish packaging of spliced RNAs [8,9]. Therefore, a possible explanation for the efficient Rev-mediated packaging of Msd1-sa5 is the lack of such a competition between the different transcripts of VHenv all of which do not contain the full-length encapsidation signal (figure 1 and figure 4). In contrast to the situation after transfection of VHenv, VHgenomic generates two transcripts, the unspliced and the singly-spliced SD1-SA5 RNA, which are exported to the cytoplasm by Rev. Since the unspliced transcript contains the full-length encapsidation signal that is truncated after splicing in the singly-spliced transcript, the presence of the unspliced RNA could limit encapsidation of the singly-spliced SD1-SA5 RNA. Encapsidation efficiencies of lentiviral vector transcripts lacking the RRE did not vary significantly with or without Rev (figure 4, red diamonds). A 2-fold increase was observed in the presence of Rev for these RNAs. It is possible that this small stimulatory trend is mediated by binding of Rev to the first RNA stem loop in the encapsidation signal present in all viral transcripts [28]. Surprisingly, encapsidation of the fully-spliced lentiviral vector transcriptswithout RRE was highly efficient in our experiments (figure 4), while fully-spliced transcripts of wild type HIV are poorly packaged [9]. Furthermore, the amount of virion-associated genomic RNA in comparison to spliced RNAs is 20 to 40-fold higher in wild type HIV particles [10,29,30]. In our experiments this effect was smaller ranging from 5-fold for fully-spliced to 17fold for singly-spliced RNAs (figure 3B). These facts demonstrate that the conditions in our experiments differed in some aspects from the wild type situation. A possible reason for these discrepancies could be that not all cells are cotransfected with every plasmid used. Cells transfected only with the Revindependent gag/gagpol expression plasmid together with the lentiviral vector but not with the rev expression plasmid could lead to particles containing mainly fully-spliced RNAs, because packaging of Rev-dependent singly-spliced and unspliced RNAs would be impaired. In addition, high intracellular RNA levels obtained after transient transfection of HEK293T cells with the CMV promoter driven lentiviral vectors in the presence of Tat are believed to reduce the specificity of the encapsidation process [5]. Furthermore, utilization of the Rev and Tat-independent gag/ gagpol expression plasmid could lead to increased packaging of fully-spliced RNA. In the HIV replication cycle the viral proteins Tat and Rev are expressed from multiply-spliced transcripts and have to accumulate to a certain threshold level to allow subsequent expression of genes from partially and unspliced RNAs [5,31?4]. Additionally, the viral protein Gag/GagPol is produced from the unspliced transcript. These facts guarantee a spatial and temporal regulated appearance of Gag and genomic RNA at the same site in the cytoplasm late in the replication cycle. Most likely this concerted gene expression pattern al.