Smith C J, Liechty M C, Rasmussen J L, Macrina F L
Basic Life Sci. 1985;30:555-70. doi: 10.1007/978-1-4613-2447-8_39.
Results presented in this paper have shown that a widely distributed LM-resistance determinant is present on at least 3 distinct Bacteroides R-plasmids. In fact these plasmids bear no homology outside of the defined regions implicated in the LM resistance. This resistance determinant on pBF4, pBFTM10, and pBI136 is located within DNA segments bounded on each side by a directly repeated sequence of more than 500 bp. The intervening sequences of these 3 elements are variable, and range in size from about 3.7 kb to 7.2 kb (Fig. 7). Apart from the EcoRI/AvaI restriction sites which characterize the repeated sequence, there is a notable lack of common restriction sites within these elements. These results suggest that the elements do possess a certain degree of structural similarity but significant evolutionary divergence has occurred. The presence and location of the directly repeated sequences, their association with specific deletions, and their association with an antibiotic-resistance determinant, are features common to many antibiotic-resistance transposons described for other prokaryotes. In addition, these elements are highly mobile, being found on a number of R-plasmids. The unique relationship between pBF4, pBI106, and pBI136 described here is a clear indication of the potential for these DNA sequences to move from one molecule to another. Given the extensive dissemination and the genetic and structural characteristics described above, it seems likely that the LM-resistance determinant is carried on transposon-like elements present in pBF4, pBFTM10, and pBI136. However, further experimentation will be necessary to document the transposition event. Bacteroides strains such as B. fragilis V503, possess a transmissible LM-resistance determinant which does not appear to be associated with detectable extrachromosomal elements (5,9,10). Presently, a number of strains of this type have been found over a wide geographic area. The LM-resistance genes associated with these strains are apparently similar to the one carried on the Bacteroides R-plasmids because homology between the 2 has been observed. However, it is important to note that within the limits of Southern filter blot hybridization, neither V503 nor its transconjugants possess the directly repeated sequence found on the LM-resistance plasmids (Fig. 8). The elusive nature of the V503 LM-resistance elements presents an intriguing problem. One model that has been proposed is that these resistance determinants reside on a conjugative transposon similar to Tn916 of Streptococcus faecalis (3).(ABSTRACT TRUNCATED AT 400 WORDS)
本文展示的结果表明，一种广泛分布的林可霉素抗性决定簇存在于至少3种不同的拟杆菌R质粒上。事实上，这些质粒在与林可霉素抗性相关的特定区域之外没有同源性。pBF4、pBFTM10和pBI136上的这种抗性决定簇位于两侧由超过500 bp的直接重复序列界定的DNA片段内。这3个元件的间隔序列是可变的，大小从约3.7 kb到7.2 kb不等（图7）。除了表征重复序列的EcoRI/AvaI限制性酶切位点外，这些元件内明显缺乏共同的限制性酶切位点。这些结果表明这些元件确实具有一定程度的结构相似性，但已经发生了显著的进化分歧。直接重复序列的存在和位置、它们与特定缺失的关联以及它们与抗生素抗性决定簇的关联，是许多针对其他原核生物描述的抗生素抗性转座子共有的特征。此外，这些元件具有高度的移动性，存在于许多R质粒上。这里描述的pBF4、pBI106和pBI136之间独特的关系清楚地表明了这些DNA序列从一个分子转移到另一个分子的可能性。鉴于上述广泛传播以及遗传和结构特征，林可霉素抗性决定簇似乎由存在于pBF4、pBFTM10和pBI136中的类转座子元件携带。然而，需要进一步的实验来证明转座事件。脆弱拟杆菌V503等拟杆菌菌株具有可传递的林可霉素抗性决定簇，似乎与可检测到的染色体外元件无关（5,9,10）。目前，在广泛的地理区域内发现了许多这种类型的菌株。与这些菌株相关的林可霉素抗性基因显然与拟杆菌R质粒上携带的基因相似，因为已观察到两者之间的同源性。然而，重要的是要注意，在Southern印迹杂交的限度内，V503及其转接合子均不具有在林可霉素抗性质粒上发现的直接重复序列（图8）。V503林可霉素抗性元件难以捉摸的性质提出了一个有趣的问题。已经提出的一种模型是，这些抗性决定簇存在于类似于粪肠球菌Tn916的接合转座子上（3）。（摘要截短至400字）
