Sanderson K E, Liu S L
Salmonella Genetic Stock Centre, Department of Biological Sciences, University of Calgary, Alberta, Canada.
Electrophoresis. 1998 Apr;19(4):569-72. doi: 10.1002/elps.1150190417.
Early genetic studies showed conservation of gene order in the enteric bacteria. Two recent methods using pulsed field gel electrophoresis (PFGE) to determine the physical map of the genome are: (i) partial digestion with the endonuclease I-CeuI, which digests the DNA of bacteria in the rrn operon for rRNA (ribosomal RNA), thus establishing the "rrn genomic skeleton" (the size in kbp of the intervals between rRNA operons); (ii) analysis of XbaI and B1nI sites within Tn10 insertions in the chromosome. The order of I-CeuI fragments, which is ABCDEFG in S. typhimurium LT2 and E. coli K-12, was found to be conserved in most Salmonella species, most of which grow in many hosts (host-generalists). However, in S. typhi, S. paratyphi C, S. gallinarum, and S. pullorum, species which are host-specialized, these fragments are rearranged, due to homologous recombination between the rrn operons, resulting in translocations and inversions. Inversions and translocations not involving the rrn operons are seldom detected except for inversions over the TER (termination of replication) region. Additive genetic changes (due to lateral transfer resulting in insertion of nonhomologous DNA) have resulted in "loops" containing blocks of DNA which provide new genes to specific strains, thus driving rapid evolution of new traits.
早期的遗传学研究表明肠道细菌中基因顺序具有保守性。最近有两种利用脉冲场凝胶电泳（PFGE）来确定基因组物理图谱的方法：（i）用核酸内切酶I-CeuI进行部分消化，该酶可消化rRNA（核糖体RNA）的rrn操纵子中的细菌DNA，从而建立“rrn基因组骨架”（rRNA操纵子之间间隔的千碱基对大小）；（ii）分析染色体上Tn10插入位点内的XbaI和B1nI位点。在鼠伤寒沙门氏菌LT2和大肠杆菌K-12中，I-CeuI片段的顺序为ABCDEFG，在大多数沙门氏菌物种中发现该顺序是保守的，其中大多数能在多种宿主中生长（宿主泛化型）。然而，在宿主专一性的伤寒沙门氏菌、副伤寒沙门氏菌C、鸡沙门氏菌和鸡白痢沙门氏菌中，由于rrn操纵子之间的同源重组，这些片段发生了重排，导致易位和倒位。除了复制终止（TER）区域的倒位外，很少检测到不涉及rrn操纵子的倒位和易位。累加性遗传变化（由于横向转移导致非同源DNA插入）产生了包含DNA片段的“环”，这些片段为特定菌株提供了新基因，从而推动了新性状的快速进化。
