Ryder T B, Davidson D B, Rosen J I, Ohtsubo E, Ohtsubo H
Gene. 1982 Mar;17(3):299-310. doi: 10.1016/0378-1119(82)90146-9.
Plasmid Rsc13, a small derivative of the plasmid R1, contains a region necessary for replication as well as a complete copy (4957 bp) of the ampicillin resistance transposon, Tn3. We determined the nucleotide sequence of the replication region of Rsc13 to be 2937 bp and then compared this region (designated the 2.9-kb region) to the analogous region of pSM1, a small derivative of the plasmid R100 which has common ancestry with R1. Rsc13 and pSM1 were 96% homologous in this 2.9-kb region except for a discrete region of about 250 bp which showed only 44% homology. The sequence and distribution of nucleotide substitutions between Rsc13 and pSM1 supported a map of possible genes and sites which have previously been seen in the replication region of Rsc13 and pSM1 which showed only 44% homology. Analysis of the amino acid sequence and predicted conformation of the two RepA2 polypeptides, however, suggested that they were very similar. We proposed that the repA2 region of R1 and R100 was replaced by a substitution of a short DNA segment from another plasmid which was evolutionarily related to R1 and R100 but had more divergence. This event may have been mediated by a mechanism similar to that of gene conversion as described in eukaryotic systems.
质粒Rsc13是质粒R1的一个小衍生物，它包含复制所需的区域以及氨苄青霉素抗性转座子Tn3的一个完整拷贝（4957碱基对）。我们确定Rsc13复制区域的核苷酸序列为2937碱基对，然后将该区域（称为2.9千碱基区域）与pSM1的类似区域进行比较，pSM1是质粒R100的一个小衍生物，与R1有共同的祖先。除了一个约250碱基对的离散区域显示只有44%的同源性外，Rsc13和pSM1在这个2.9千碱基区域的同源性为96%。Rsc13和pSM1之间核苷酸取代的序列和分布支持了一个可能的基因和位点图谱，这些基因和位点以前在Rsc13和pSM1的复制区域中出现过，同源性仅为44%。然而，对两种RepA2多肽的氨基酸序列和预测构象的分析表明它们非常相似。我们提出，R1和R100的repA2区域被来自另一个与R1和R100进化相关但差异更大的质粒的短DNA片段取代。这一事件可能是由一种类似于真核系统中描述的基因转换机制介导的。
