Stokes H W, Tomaras C, Parsons Y, Hall R M
School of Biological Sciences, Macquarie University, Sydney NSW.
Plasmid. 1993 Jul;30(1):39-50. doi: 10.1006/plas.1993.1032.
Integrons are genetic elements which are capable of acquiring genes by site-specific recombination. The most common integron structure consists of two conserved segments flanking a variable region where many different antibiotic resistance genes have been found. The integrons In6 and In7, present in the plasmids pSa and pDGO100, respectively, are unusual in that they include a duplication of the sulI gene which is located within the integron 3'-conserved segment. To further investigate the structure of these integrons, the DNA sequence of the segment located between the two sulI genes was determined. In In7 this segment is 2822 bases long and includes a trimethoprim resistance gene, dhfrX, at one end. The corresponding region in In6 is 4.5 kb and is nearly identical to the In7 segment over the first 2105 bases. In the region unique to In6, a cat gene, conferring chloramphenicol resistance, has replaced the dhfrX gene of In7. This location thus represents a second variable region where different antibiotic resistance genes are found, but the way in which genes become associated with this second variable region is not known. The overall similarity of the structures of In6 and In7 suggests that the additional DNA segments found in these integrons have a common origin, and a possible mechanism for the origin of integrons with partial 3'-conserved segment duplications is presented.
整合子是能够通过位点特异性重组获取基因的遗传元件。最常见的整合子结构由两个保守区段组成，两侧为可变区，在该可变区已发现许多不同的抗生素抗性基因。分别存在于质粒pSa和pDGO100中的整合子In6和In7不同寻常之处在于，它们包含位于整合子3'保守区段内的sulI基因的重复序列。为了进一步研究这些整合子的结构，测定了两个sulI基因之间区段的DNA序列。在In7中，该区段长2822个碱基，一端包含一个甲氧苄啶抗性基因dhfrX。In6中的相应区域为4.5 kb，在前2105个碱基上与In7区段几乎相同。在In6特有的区域，一个赋予氯霉素抗性的cat基因取代了In7的dhfrX基因。因此，这个位置代表了另一个可变区，在该区域发现了不同的抗生素抗性基因，但尚不清楚基因与这个第二可变区关联的方式。In6和In7结构的总体相似性表明，在这些整合子中发现的额外DNA区段有共同的起源，并提出了具有部分3'保守区段重复的整合子起源的一种可能机制。
