Hua S S, Markovitz A
J Bacteriol. 1972 Jun;110(3):1089-99. doi: 10.1128/jb.110.3.1089-1099.1972.
Previous studies showed that nonsense mutations in either of two genes (capR or capS) or an undefined mutation in a third gene (capT) led to pleiotropic effects: (i) increased capsular polysaccharide synthesis (mucoid phenotype); (ii) increased synthesis of enzymes specified by at least four spatially separated operons involved in synthesis of capsular polysaccharide including the product of the galE gene, UDP-galactose-4-epimerase (EC 5.1.3.2) in capR mutants. The present study demonstrated that the entire galactose (gal) operon (galE, galT, and galK) is derepressed by mutations in either the capR or the capT genes, but not by mutation in capS. Double mutants (capR9 capT) were no more derepressed than the capR9 mutant, indicating that capR9 and capT regulate the gal operon via a common pathway. Isogenic double mutants containing either galR(+), galR(-), galR(s), or galO(c) in combination with either capR(+) or capR9 were prepared and analyzed for enzymes of the gal operon. The results demonstrated that capR9 caused derepression as compared to capR(+) in all of the combinations. Strains with a galR(s) mutation are not induced, for the gal operon, by any galactose compound including d-fucose, and this was confirmed in the present study using d-fucose. Nevertheless, the derepression of galR(s) capR9 compared to galR(s) capR(+) was four- to sixfold. The same derepression was observed when galR(+)capR9 was compared to galR(+)capR(+). The data eliminate the explanation that internal induction of the gal operon by a galactose derivative was causing increased gal operon enzyme synthesis in capR or capT mutants. Furthermore, the same data suggest that the galR and capR genes are acting independently to derepress the gal operon. A modified model for the structure of the gal operon is proposed to explain these results. The new feature of the model is that two operator sites are suggested, one to combine with the galR repressor and one to combine with the capR repressor.
先前的研究表明，两个基因（capR或capS）中的任何一个发生无义突变，或者第三个基因（capT）发生未明确的突变，都会导致多效性效应：（i）荚膜多糖合成增加（黏液样表型）；（ii）至少四个参与荚膜多糖合成的空间上分离的操纵子所指定的酶的合成增加，包括capR突变体中galE基因的产物UDP-半乳糖-4-表异构酶（EC 5.1.3.2）。本研究表明，capR或capT基因的突变会使整个半乳糖（gal）操纵子（galE、galT和galK）去阻遏，但capS基因的突变则不会。双突变体（capR9 capT）的去阻遏程度并不比capR9突变体更高，这表明capR9和capT通过共同途径调节gal操纵子。构建了含有galR(+)、galR(-)、galR(s)或galO(c)与capR(+)或capR9组合的同基因双突变体，并对gal操纵子的酶进行了分析。结果表明，在所有组合中，与capR(+)相比，capR9都会导致去阻遏。具有galR(s)突变的菌株不会被任何半乳糖化合物（包括d-岩藻糖）诱导表达gal操纵子，本研究使用d-岩藻糖对此进行了证实。然而，与galR(s) capR(+)相比，galR(s) capR9的去阻遏程度为四到六倍。当比较galR(+)capR9和galR(+)capR(+)时，也观察到了相同程度的去阻遏。这些数据排除了半乳糖衍生物对gal操纵子的内部诱导导致capR或capT突变体中gal操纵子酶合成增加的解释。此外，相同的数据表明galR和capR基因独立发挥作用使gal操纵子去阻遏。为了解释这些结果，提出了一种gal操纵子结构的改进模型。该模型的新特点是提出了两个操纵位点，一个与galR阻遏物结合，另一个与capR阻遏物结合。
