The process of genome packaging in most of viruses is poorly understood, notably the role of the genome itself in the nucleocapsid structure. For simple icosahedral single-stranded RNA viruses, the branched topology due to the RNA secondary structure is thought to lower the free energy required to complete a virion. We investigate the structure of nucleocapsids packaging RNA segments with various degrees of compactness by small-angle x-ray scattering and cryotransmission electron microscopy. The structural differences are mild even though compact RNA segments lead on average to better-ordered and more uniform particles across the sample. Numerical calculations confirm that the free energy is lowered for the RNA segments displaying the larger number of branch points. The effect is, however, opposite with synthetic polyelectrolytes, in which a star topology gives rise to more disorder in the capsids than a linear topology. If RNA compactness and size account in part for the proper assembly of the nucleocapsid and the genome selectivity, other factors most likely related to the host cell environment during viral assembly must come into play as well.

大多数病毒的基因组包装过程知之甚少，特别是基因组本身在核衣壳结构中的作用。对于简单的二十面体单链 RNA 病毒，由于 RNA 二级结构的分支拓扑被认为降低了完成病毒体所需的自由能。我们通过小角度 X 射线散射和低温透射电子显微镜研究了包装具有不同紧密度的 RNA 片段的核衣壳的结构。尽管紧凑的 RNA 片段平均导致样品中的颗粒更有序和更均匀，但结构差异是轻微的。数值计算证实，显示更多分支点的 RNA 片段的自由能降低。然而，这种效果与合成聚电解质相反，其中星形拓扑比线性拓扑在衣壳中产生更多的无序。如果 RNA 紧凑性和大小部分解释了核衣壳的正确组装和基因组选择性，那么在病毒组装过程中最可能与宿主细胞环境相关的其他因素也必须发挥作用。