Griffiths A J
Botany Department, University of British Columbia, Vancouver, Canada.
Microbiol Rev. 1995 Dec;59(4):673-85. doi: 10.1128/mr.59.4.673-685.1995.
Among eukaryotes, plasmids have been found in fungi and plants but not in animals. Most plasmids are mitochondrial. In filamentous fungi, plasmids are commonly encountered in isolates from natural populations. Individual populations may show a predominance of one type, but some plasmids have a global distribution, often crossing species boundaries. Surveys have shown that strains can contain more than one type of plasmid and that different types appear to be distributed independently. In crosses, plasmids are generally inherited maternally. Horizontal transmission is by cell contact. Circular plasmids are common only in Neurospora spp., but linear plasmids have been found in many fungi. Circular plasmids have one open reading frame (ORF) coding for a DNA polymerase or a reverse transcriptase. Linear plasmids generally have two ORFs, coding for presumptive DNA and RNA polymerases with amino acid motifs showing homology to viral polymerases. Plasmids often attain a high copy number, in excess of that of mitochondrial DNA. Linear plasmids have a protein attached to their 5' end, and this is presumed to act as a replication primer. Most plasmids are neutral passengers, but several linear plasmids integrate into mitochondrial DNA, causing death of the host culture. Inferred amino acid sequences of linear plasmid ORFs have been used to plot phylogenetic trees, which show a fair concordance with conventional trees. The circular Neurospora plasmids have replication systems that seem to be evolutionary intermediates between the RNA and the DNA worlds.
在真核生物中，已在真菌和植物中发现质粒，而在动物中未发现。大多数质粒存在于线粒体中。在丝状真菌中，天然群体分离株中普遍存在质粒。单个群体可能以某一种类型为主，但有些质粒具有全球分布，常常跨越物种界限。调查表明，菌株可能含有不止一种类型的质粒，而且不同类型的质粒似乎是独立分布的。在杂交中，质粒通常通过母系遗传。水平传播是通过细胞接触实现的。环状质粒仅在脉孢菌属中常见，但线性质粒已在许多真菌中发现。环状质粒有一个开放阅读框（ORF），编码一种DNA聚合酶或逆转录酶。线性质粒一般有两个ORF，编码推测的DNA和RNA聚合酶，其氨基酸基序与病毒聚合酶具有同源性。质粒常常具有高拷贝数，超过线粒体DNA的拷贝数。线性质粒在其5'端附着有一个蛋白质，推测该蛋白质作为复制引物起作用。大多数质粒是中性的过客，但一些线性质粒会整合到线粒体DNA中，导致宿主培养物死亡。线性质粒ORF的推断氨基酸序列已被用于绘制系统发育树，这些树与传统树显示出相当的一致性。脉孢菌的环状质粒具有的复制系统似乎是RNA世界和DNA世界之间的进化中间体。
