Volkert F C, Wu L C, Fisher P A, Broach J R
Basic Life Sci. 1986;40:375-96. doi: 10.1007/978-1-4684-5251-8_29.
The multicopy yeast plasmid 2-micron circle uses a number of strategies to insure its persistence in its host. The plasmid confers no selective phenotype to the cell in which it is resident. Nonetheless, the plasmid is lost at less than 1 per 10(5) cell divisions during continuous exponential growth. We have determined that the plasmid persists at least in part due to the ability of the plasmid to amplify its mean copy number when its cellular copy level is low and to distribute plasmid molecules equally between mother and daughter cells at mitosis. We have found that amplification of plasmid copy number occurs by a novel mechanism in which site-specific recombination induces a transient shift in the mode of replication from theta to rolling circle. Equitable partitioning of plasmid molecules requires plasmid-encoded proteins and a centromere-like segment on the plasmid. We have accumulated evidence consistent with a model of partitioning in which the partitioning proteins form a transnuclear structure that is responsible for distributing plasmid molecules throughout the nucleus prior to cell division. In this chapter we describe evidence supporting the existence and mode of action of these two plasmid strategies and discuss the extent to which these strategies may be a pervasive facet of the biology of eukaryotic extrachromosomal elements.
多拷贝酵母质粒2-微米环利用多种策略确保其在宿主中的持久性。该质粒不会赋予其所在细胞任何选择性表型。尽管如此，在连续指数生长期间，该质粒在每10⁵次细胞分裂中丢失不到1次。我们已经确定，该质粒至少部分地由于其在细胞拷贝水平较低时扩增其平均拷贝数以及在有丝分裂时将质粒分子在母细胞和子细胞之间均匀分配的能力而得以持续存在。我们发现，质粒拷贝数的扩增通过一种新机制发生，即位点特异性重组诱导复制模式从θ型转变为滚环型。质粒分子的公平分配需要质粒编码的蛋白质和质粒上类似着丝粒的片段。我们已经积累了与一种分配模型一致的证据，在该模型中，分配蛋白形成一种跨核结构，负责在细胞分裂前将质粒分子分布在整个细胞核中。在本章中，我们描述了支持这两种质粒策略的存在及其作用方式的证据，并讨论了这些策略在多大程度上可能是真核染色体外元件生物学中普遍存在的一个方面。
