Reznikoff W S, Michels C A, Cooper T G, Silverstone A E, Magasanik B
J Bacteriol. 1974 Mar;117(3):1231-9. doi: 10.1128/jb.117.3.1231-1239.1974.
Different levels of beta-galactosidase are found in various trp-lac fusion strains. These levels of beta-galactosidase fall within a 60-fold range. The amount of thiogalactoside transacetylase activity detected in these same strains only varies 10-fold and is found in amounts greater than those predicted from the beta-galactosidase levels. The observation that the beta-galactosidase and thiogalactoside transacetylase levels are not directly proportional, that the lacZ messenger ribonucleic acid (mRNA) levels are not proportional to the beta-galactosidase activity, that, at least for the one fusion strain tested, the SuA polarity suppressor does not affect the beta-galactosidase level, and that, in all but one strain, the beta-galactosidase activity appears to reside in normal beta-galactosidase molecules suggests that the disproportionately low production of beta-galactosidase is due to a decrease in the frequency of translation initiation of lacZ mRNA in these strains. Several mechanisms are proposed to explain this decrease. Some possible bases for the disproportional production of beta-galactosidase and thiogalactoside transacetylase are also described. The preferred explanation for these disproportional enzyme levels is that only a fraction of the full complement of ribosomes need initiate translation at lacZ for the functional synthesis of lac mRNA to occur and that once the lac ribonucleic acid is made a full complement of ribosomes can bind at internal translation initiation sites at Y and A.
在各种色氨酸-乳糖操纵子融合菌株中发现了不同水平的β-半乳糖苷酶。这些β-半乳糖苷酶水平的范围达60倍。在这些相同菌株中检测到的硫代半乳糖苷转乙酰酶活性的量仅相差10倍，并且其含量高于根据β-半乳糖苷酶水平预测的量。β-半乳糖苷酶和硫代半乳糖苷转乙酰酶水平不成正比、乳糖Z信使核糖核酸（mRNA）水平与β-半乳糖苷酶活性不成正比、至少对于所测试的一个融合菌株而言，SuA极性抑制因子不影响β-半乳糖苷酶水平，以及除一个菌株外，在所有菌株中β-半乳糖苷酶活性似乎存在于正常的β-半乳糖苷酶分子中，这些观察结果表明，β-半乳糖苷酶产量异常低是由于这些菌株中乳糖Z mRNA翻译起始频率降低所致。提出了几种机制来解释这种降低。还描述了β-半乳糖苷酶和硫代半乳糖苷转乙酰酶产量不成比例的一些可能原因。对于这些不成比例的酶水平，最合理的解释是，只有一部分完整的核糖体需要在乳糖Z处起始翻译才能发生乳糖mRNA的功能性合成，并且一旦合成了乳糖核糖核酸，完整的核糖体就可以结合在Y和A处的内部翻译起始位点上。
