Both the 5′ and 3′ LTRs of FIV contain minor RNA encapsidation determinants compared to the two core packaging determinants within the 5′ untranslated region and gag

This study was undertaken to address the role of feline immunodeficiency virus (FIV) long terminal repeats (LTR) as potential packaging determinants. A number of studies in the recent past have clearly demonstrated that the core packaging determinants of FIV reside within at least two distinct regions at the 5′ end of the viral genome, from R in the 5′ LTR to ∼150 bp within the 5′ untranslated region (5′ UTR) and within the first 100 bp of gag; however, there have been conflicting observations as to the role of the LTR regions in packaging and whether they contain the principal packaging determinants of FIV. Using a semi-quantitative RT-PCR approach on heterologous non-viral vector RNAs in an in vivo packaging assay, this study demonstrates that the principal packaging determinants of FIV reside within the first 150 bp of 5′ UTR and 100 bp of gag (the two core regions) and not the viral 5′ LTR. Furthermore, it shows that in addition to the 5′ LTR, the 3′ LTR also contains packaging determinants, but of a less significant nature compared to the core packaging determinants. This study defines the relative contribution of the various regions implicated in FIV genomic RNA packaging, and reveals that like other primate lentiviruses, the packaging determinants of FIV are multipartite and spread out, an observation that has implications for safer and more streamlined design of FIV-based gene transfer vectors.