Chandler V L, Talbert L E, Raymond F
Institute of Molecular Biology, University of Oregon, Eugene 97403.
Genetics. 1988 Aug;119(4):951-8. doi: 10.1093/genetics/119.4.951.
The increased mutation rate of Mutator stocks of maize has been shown to be the result of transposition of Mu elements. One element, Mu1, is present in 10-60 copies in Mutator stocks and approximately 0-3 copies in non-Mutator stocks. The sequence, structure and genomic distribution of an intact Mu1 element cloned from the non-Mutator inbred line B37 has been determined. The sequence of this element, termed Mu1.4-B37, is identical to Mu1 and it is flanked by 9-bp direct repeats indicative of a target site duplication. Mu1.4-B37 is not in the same genomic location in all stocks, which further suggests that it transposed into its genomic location in B37. We previously reported that in genomic DNA this element is modified such that certain methylation-sensitive restriction enzymes will not cut sites within the element. This is similar to that observed for Mu elements in Mutator stocks that have lost activity. We report herein that the Mu1.4-B37 element loses its modification and becomes accessible to digestion when placed in an active Mutator stock by genetic crosses. This suggests that factors conditioning unmodified elements are dominant in the initial cross between Mutator and non-Mutator stocks. In F2 individuals that have subsequently lost Mutator activity the Mu1.4-B37 element again becomes modified as do most of the Mu elements in the stock. Thus, the modification state of the Mu1.4-B37 element and the other Mu1-like elements correlates with Mutator activity. We hypothesize that factor(s) within an active Mutator stock may inhibit the modification of Mu elements, and that this activity is missing in non-Mutator stocks and may become limiting in certain Mutator stocks resulting in DNA modification.
玉米Mutator株系突变率的增加已被证明是Mu元件转座的结果。一种元件Mu1，在Mutator株系中存在10 - 60个拷贝，在非Mutator株系中约有0 - 3个拷贝。已确定从非Mutator自交系B37克隆的完整Mu1元件的序列、结构和基因组分布。这个元件的序列，称为Mu1.4 - B37，与Mu1相同，其两侧有9个碱基对的直接重复序列，表明存在靶位点重复。Mu1.4 - B37在所有株系中的基因组位置并不相同，这进一步表明它是转座到B37的基因组位置中的。我们之前报道过，在基因组DNA中，这个元件发生了修饰，使得某些对甲基化敏感的限制酶不会切割该元件内的位点。这与在已失去活性的Mutator株系中的Mu元件所观察到的情况相似。我们在此报告，当通过遗传杂交将Mu1.4 - B37元件置于活性Mutator株系中时，它会失去修饰并变得可被酶切。这表明在Mutator株系和非Mutator株系的初始杂交中，决定未修饰元件的因子是显性的。在随后失去Mutator活性的F2个体中，Mu1.4 - B37元件再次像株系中的大多数Mu元件一样发生修饰。因此，Mu1.4 - B37元件和其他类似Mu1的元件的修饰状态与Mutator活性相关。我们推测，活性Mutator株系中的因子可能会抑制Mu元件的修饰，并且这种活性在非Mutator株系中不存在，在某些Mutator株系中可能会受到限制，从而导致DNA修饰。
