Koffel-Schwartz N, Fuchs R P
Institut de biologie moléculaire et cellulaire du CNRS, Strasbourg, France.
Mol Gen Genet. 1989 Jan;215(2):306-11. doi: 10.1007/BF00339733.
In a previous study, the forward mutation spectrum induced by the chemical carcinogen N-acetoxy-N-2-acetylaminofluorene was determined (Koffel-Schwartz et al. 1984). It was found that 90% of the induced mutations are frameshift mutations located within specific sequences (mutation hot spots). Two classes of mutation hot spots were found: (i) -1 frameshift mutations occurring within runs of guanines (i.e. GGGG----GGG; (ii) -2 frameshift mutations occurring within the NarI recognition sequence (GGCGCC----GGCC). In the present work, we further investigate the genetic requirements of these frameshift events by using specific reversion assays. Like UV-induced mutagenesis, frameshift mutations occurring within runs of G's (also referred to as the "slippage pathway") require the activated form of the RecA protein (RecA*). On the other hand, frameshift mutations occurring at the NarI site (the "NarI mutation pathway") require a LexA-controlled function(s) that is not UmuDC. The LexA-controlled gene(s) that is (are) involved in this pathway remain to be identified. Moreover, this pathway does not require RecA* for the proteolytic processing of a protein other than LexA (like the cleavage of UmuD in UV-induced mutagenesis). An "additional" role of RecA can be defined as follows: (i) The non-activated form of the RecA protein acts as an inhibitor in the NarI mutation pathway. (ii) This inhibition is relieved upon activation of RecA by UV irradiation of the bacteria. (iii) A recA deletion mutant is totally proficient in the NarI mutation pathway provided the SOS system is derepressed [lexA (Def) allele]. Therefore, RecA does not actively participate in the fixation of the mutation. A molecular model for this "additional" role of RecA is proposed.
在之前的一项研究中，测定了化学致癌物N-乙酰氧基-N-2-乙酰氨基芴诱导的正向突变谱（Koffel-Schwartz等人，1984年）。结果发现，90%的诱导突变是位于特定序列（突变热点）内的移码突变。发现了两类突变热点：（i）在鸟嘌呤串内发生的-1移码突变（即GGGG----GGG）；（ii）在NarI识别序列（GGCGCC----GGCC）内发生的-2移码突变。在本研究中，我们通过使用特定的回复突变分析进一步研究这些移码事件的遗传要求。与紫外线诱导的诱变一样，在G串内发生的移码突变（也称为“滑动途径”）需要RecA蛋白的活化形式（RecA*）。另一方面，在NarI位点发生的移码突变（“NarI突变途径”）需要一种由LexA控制的功能，该功能不是UmuDC。参与该途径的LexA控制基因仍有待确定。此外，该途径不需要RecA*对LexA以外的蛋白质进行蛋白水解加工（如紫外线诱导诱变中UmuD的切割）。RecA的“额外”作用可定义如下：（i）RecA蛋白的非活化形式在NarI突变途径中起抑制剂作用。（ii）通过对细菌进行紫外线照射激活RecA后，这种抑制作用会解除。（iii）如果SOS系统被去阻遏[lexA（Def）等位基因]，recA缺失突变体在NarI突变途径中完全正常。因此，RecA并不积极参与突变的固定。本文提出了RecA这种“额外”作用的分子模型。
