Lindahl T, Demple B, Robins P
EMBO J. 1982;1(11):1359-63. doi: 10.1002/j.1460-2075.1982.tb01323.x.
The O6-methylguanine-DNA methyltransferase of Escherichia coli acts rapidly and stoichiometrically to convert a mutagenic O6-methylguanine residue in DNA to unsubstituted guanine. Even at low protein concentrations and in the absence of any cofactors, the transfer of a methyl group to one of the protein's own cysteine residues occurs in less than 2 s at 37 degrees C. The entire kinetic process can be followed experimentally at 5 degrees C. Formation of S-methylcysteine in the protein is accompanied by loss of activity and accounts for the exceptional suicide kinetics of this enzyme as well as for the sharp saturation of O6-methylguanine repair observed in vivo. The enzyme can remove greater than 98% of the methyl groups from O6-methylguanine present in alkylated DNA, but leaves N-alkylated purines untouched. Single-stranded DNA containing O6-methylguanine is a poor substrate, with the methyl transfer occurring at approximately 0.1% of the rate for duplex DNA. This latter observation may explain the high frequency of mutations induced by alkylating agents at DNA replication forks.
大肠杆菌的O6-甲基鸟嘌呤-DNA甲基转移酶能迅速且按化学计量比将DNA中具有诱变作用的O6-甲基鸟嘌呤残基转化为未被取代的鸟嘌呤。即使在低蛋白浓度且不存在任何辅因子的情况下，在37℃时，甲基向该蛋白自身的一个半胱氨酸残基转移的过程在不到2秒内即可发生。整个动力学过程在5℃时可通过实验进行追踪。蛋白中S-甲基半胱氨酸的形成伴随着活性的丧失，这解释了该酶独特的自杀动力学以及体内观察到的O6-甲基鸟嘌呤修复的急剧饱和现象。该酶能从烷基化DNA中存在的O6-甲基鸟嘌呤去除超过98%的甲基，但对N-烷基化嘌呤则无作用。含有O6-甲基鸟嘌呤的单链DNA是一种较差的底物，甲基转移的速率约为双链DNA的0.1%。后一观察结果可能解释了烷基化剂在DNA复制叉处诱导突变的高频率现象。
