For gene duplication to be maintained, particularly in the small genomes of RNA viruses, this should offer some advantages. We have investigated the functions of a small protein termed VPg or 3B, which acts as a primer in the replication of foot-and-mouth disease virus (FMDV). Many related picornaviruses encode a single copy but uniquely the FMDV genome includes three (nonidentical) copies of the 3B coding region. Using sub-genomic replicons incorporating nonfunctional 3Bs and 3B fusion products in competition and complementation assays, we investigated the contributions of individual 3Bs to replication and the structural requirements for functionality. We showed that a free N-terminus is required for 3B to function as a primer and although a single 3B can support genome replication, additional copies provide a competitive advantage. However, a fourth copy confers no further advantage. Furthermore, we find that a minimum of two 3Bs is necessary for trans replication of FMDV replicons, which is unlike other picornaviruses where a single 3B can be used for both cis and trans replication. Our data are consistent with a model in which 3B copy number expansion within the FMDV genome has allowed evolution of separate cis and trans acting functions, providing selective pressure to maintain multiple copies of 3B.

中文翻译：

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FMDV 基因组 3B 编码区三重复制的功能优势

为了维持基因复制，特别是在 RNA 病毒的小基因组中，这应该提供一些优势。我们研究了一种称为 VPg 或 3B 的小蛋白质的功能，它在口蹄疫病毒 (FMDV) 的复制中充当引物。许多相关的小核糖核酸病毒编码单个拷贝，但 FMDV 基因组包含 3B 编码区的三个（不相同）拷贝。在竞争和互补分析中使用包含非功能性 3B 和 3B 融合产物的亚基因组复制子，我们研究了单个 3B 对复制的贡献和功能的结构要求。我们表明，3B 需要一个自由的 N 端作为引物，虽然单个 3B 可以支持基因组复制，但额外的拷贝提供了竞争优势。然而，第四份副本没有进一步的优势。此外，我们发现 FMDV 复制子的反式复制至少需要两个 3B，这与其他小核糖核酸病毒不同，其中单个 3B 可用于顺式和反式复制。我们的数据与模型一致，其中 FMDV 基因组内的 3B 拷贝数扩展允许进化单独的顺式和反式作用功能，提供选择压力以维持多个 3B 拷贝。