Midgley J E
Biochem J. 1976 Feb 15;154(2):541-52. doi: 10.1042/bj1540541.
The synthesis of ribosomes was compared in rel+ and rel- strains of Escherichia coli undergoing "stepdown" in growth from glucose medium to one with lactate as principal carbon source. Two strains (CP78 and CP79), isogenic except for rel, showed similar behaviour with respect to (1) the kinetics of labelling total RNA and ribosomes with exogenous uracil, (2) the proportion of newly formed protein that could be bound with nascent rRNA in mature ribosomes, and (3) the rate of induction of enzymically active beta-galactosidase (relative to the rate of ribosome synthesis). It was concluded that, as there was no net accumulation of RNA during stepdown in either strain, rRNA turnover must be occurring at a high rate. The general features of ribosome maturation in rel+ and rel- cells were almost identical with those found in auxotrophic rel+ organisms starved of required amino acids. In both cases, there was a considerable delay in the maturation of new ribosomal particles, owing to a relative shortfall in the rate of synthesis of ribosome-associated proteins. Only about 4-5% of the total protein labelled during stepdown was capable of binding with newly formed rRNA. This compared with 3.5% for rel+ and 0.5% for rel- auxotrophs during amino acid starvation. The turnover rate for newly formed mRNA and rRNA was virtually the same in "stepped-down" rel+ and rel- strains and was similar to that of the same fraction in amino acid-starved rel+ cells. The functional lifetime of mRNA was also identical. It seems that in the rel- strain many of the characteristics typical of the isogenic rel+ strain are displayed under these conditions, at least as regards the speed of ribosome maturation and the induction of beta-galactosidase. Studies on the thermolability of the latter enzyme induced during stepdown indicate that inaccurate translation, which occurs in rel- strains starved for only a few amino acids, is less evident in this situation than in straightforward amino acid deprivation.
在从葡萄糖培养基向以乳酸作为主要碳源的培养基转变过程中经历“降阶”生长的大肠杆菌rel⁺和rel⁻菌株中，对核糖体的合成进行了比较。除rel外基因相同的两个菌株（CP78和CP79）在以下方面表现出相似行为：（1）用外源尿嘧啶标记总RNA和核糖体的动力学；（2）成熟核糖体中新形成的可与新生rRNA结合的蛋白质比例；（3）酶活性β-半乳糖苷酶的诱导速率（相对于核糖体合成速率）。得出的结论是，由于在任何一个菌株的降阶过程中RNA都没有净积累，rRNA周转必定以高速率发生。rel⁺和rel⁻细胞中核糖体成熟的一般特征与在缺乏所需氨基酸的营养缺陷型rel⁺生物体中发现的特征几乎相同。在这两种情况下，由于核糖体相关蛋白质合成速率相对不足，新核糖体颗粒的成熟都有相当大的延迟。在降阶过程中标记的总蛋白质中只有约4 - 5%能够与新形成的rRNA结合。相比之下，在氨基酸饥饿期间，rel⁺营养缺陷型为3.5%，rel⁻营养缺陷型为0.5%。在“降阶”的rel⁺和rel⁻菌株中，新形成的mRNA和rRNA的周转速率实际上是相同的，并且与氨基酸饥饿的rel⁺细胞中相同部分的周转速率相似。mRNA的功能寿命也是相同的。似乎在这些条件下，rel⁻菌株表现出许多与同基因rel⁺菌株典型的特征，至少在核糖体成熟速度和β-半乳糖苷酶的诱导方面是这样。对降阶过程中诱导的后一种酶的热稳定性研究表明，在仅缺乏少数几种氨基酸的rel⁻菌株中发生的不准确翻译，在这种情况下比在直接的氨基酸剥夺中不太明显。
