Perlman D, Stickgold R
Proc Natl Acad Sci U S A. 1977 Jun;74(6):2518-22. doi: 10.1073/pnas.74.6.2518.
The drug-resistance plasmid, NR1, is a 37-micron circular DNA molecule that contains two components: the resistance transfer factor (29 micron) carrying the transfer genes and the genes for tetracycline resistance, and the r-determinant (8 micron) carrying the genes for resistance to several other antibiotics including chloramphenicol (Cm). In Proteus mirabilis, these two components are capable of independent replication, or they may replicate as a composite molecule. When cells of P. mirabilis containing NR1 are cultured in medium containing Cm at 250 microgram/ml a growth lag of 20-35 hr ensues. During this lag, Cm induces the selective amplification of the r-determinant, including the gene for resistance to Cm. The amplification results from the excision of the r-determinant from the R plasmid, the independent replication of the r-determinant to give polymeric as well as monomeric r-determinants, and the eventual reintegration of multiple tandem copies of the r-determinant with the resistance transfer factor to form a new R plasmid with multiple copies of the r-determinant. This mechanism represents a new level of control of gene expression in bacterial systems--namely, the induction of selective gene amplification.
抗药性质粒NR1是一个37微米的环状DNA分子，它包含两个组分：带有转移基因和四环素抗性基因的抗性转移因子（29微米），以及携带对包括氯霉素（Cm）在内的其他几种抗生素抗性基因的r-决定子（8微米）。在奇异变形杆菌中，这两个组分能够独立复制，或者它们也可以作为一个复合分子进行复制。当含有NR1的奇异变形杆菌细胞在含有250微克/毫升Cm的培养基中培养时，会出现20 - 35小时的生长延迟。在此延迟期间，Cm诱导r-决定子的选择性扩增，包括对Cm抗性的基因。扩增是由于r-决定子从R质粒上切除、r-决定子独立复制产生多聚体以及单体r-决定子，最终多个串联拷贝的r-决定子与抗性转移因子重新整合形成带有多个r-决定子拷贝的新R质粒。这种机制代表了细菌系统中基因表达调控的一个新水平——即选择性基因扩增的诱导。
