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Of the myriad viruses, very few have been shown to be capable of self- assembly in vitro from purified components into infectious virus particles. One of these is Cowpea Chlorotic Mottle Virus (CCMV), an unenveloped spherical plant virus whose capsid self-assembles around its RNA genome without a packaging signal. While heterologous RNA, not just cognate viral RNA, can be packaged into individual CCMV virus-like particles (VLPs), the RNA needs to fall within a certain range of lengths. If it is too short, it is packaged into particles smaller than wild type, or with two or more RNAs per capsid. If the RNA is too long, multiple capsids assemble around one RNA, and the RNA associated with these multiplet structures is not as RNase resistant. Further, as shown in the present work, 4200 nt appears to be the limiting length of RNA that can be packaged into single RNase-resistant CCMV VLPs. We explore the extent to which “overlong” RNA can be packaged more efficiently upon the addition of spermine, a polyvalent cation whose increasing concentration has been shown to compactify RNA. Finally, we show that the capsid protein of Brome Mosaic Virus (BMV), a bromovirus closely related to CCMV, also gives rise to multiplets when it is self-assembled with the same “overlong” RNA constructs, but with different distributions of multiplets. 

Self-amplifying (sa) RNA molecules—“replicons”—derived from the genomes of positive-sense RNA viruses are receiving increasing attention as gene and vaccine delivery vehicles. This is because mRNA forms of genes of interest can be incorporated into them and strongly amplified, thereby enhancing target protein expression. In this report, we demonstrate a nonmonotonic dependence of protein expression on the mass of transfected replicon, in contrast to the usual, monotonic case of non-saRNA transfections. We lipotransfected a variety of cell lines with increasing masses of enhanced yellow fluorescent protein (eYFP) as a reporter gene in sa form and found that there is a “sweet spot” at which protein expression and cell viability are optimum. To control the varying mass of transfected replicon RNA for a given mass of Lipofectamine, the replicons were mixed with a “carrier” RNA that is neither replicated nor translated; the total mass of transfected RNA was kept constant while increasing the fraction of the replicon from zero to one. Fluorescence microscopy studies showed that the optimum protein expression and cell viability are achieved for replicon fractions as small as 1/10 of the total transfected RNA, and these results were quantified by a systematic series of flow cytometry measurements.