Smith R J, Midgley J E
Biochem J. 1973 Oct;136(2):249-57. doi: 10.1042/bj1360249.
During the inhibition of RC(str), but not RC(rel) mutants of Escherichia coli by trimethoprim the unusual nucleotides MSI (guanosine tetraphosphate, ppGpp) and MSII rapidly accumulated. The production of these nucleotides was not dependent on the addition of nucleotide base supplements to RC(str) cultures before trimethoprim, and the MSI nucleotide concentrations in non-supplemented or purine-supplemented cultures were comparable with the concentrations obtained when the cells were inhibited with l-valine (1g/l). Rifampicin rapidly decreased MSI and MSII nucleotide concentrations in trimethoprim-inhibited cultures to the basal values. Several situations were noted, in which MS nucleotide concentrations in trimethoprim-inhibited RC(str) cells could be drastically lowered without giving rise to an immediate resumption of stable RNA accumulation. If RC(str) mutants were first inhibited with trimethoprim and then given purines 15min later, MS nucleotide concentrations fell rapidly, because of a temporarily enhanced rate of accumulation of stable RNA. However, after a further 5min, RNA accumulation stopped, though MS nucleotide concentrations remained low. Also, if either glycine or methionine were added to trimethoprim-inhibited cultures supplemented with purines, RNA accumulation did not resume, though MS nucleotide concentrations rapidly declined. With both amino acids present, there was both a decline in MS nucleotide concentration and a resumption in stable RNA synthesis. These findings suggest that MSI nucleotide concentrations in trimethoprim-inhibited bacteria are not the sole factors in the control of stable RNA synthesis. It is possible that, during the period when the RC(str) cells contained high concentrations of MS nucleotides, some factor important in the MSI-mediated control of stable RNA synthesis was irreversibly inactivated. However, as antibiotics (e.g. chloramphenicol) both abolished high MS nucleotide concentrations and permitted a rapid resumption of stable RNA accumulation in the same conditions, it is more likely that an additional control mechanism has come into play.
在甲氧苄啶抑制大肠杆菌的RC(str)突变体而非RC(rel)突变体的过程中，异常核苷酸MSI（鸟苷四磷酸，ppGpp）和MSII迅速积累。这些核苷酸的产生不依赖于在甲氧苄啶处理之前向RC(str)培养物中添加核苷酸碱基补充剂，并且未补充或补充嘌呤的培养物中MSI核苷酸浓度与用L-缬氨酸（1g/l）抑制细胞时获得的浓度相当。利福平迅速将甲氧苄啶抑制的培养物中的MSI和MSII核苷酸浓度降低至基础值。注意到几种情况，在这些情况下，甲氧苄啶抑制的RC(str)细胞中的MS核苷酸浓度可大幅降低，而不会立即恢复稳定RNA积累。如果RC(str)突变体先用甲氧苄啶抑制，然后在15分钟后给予嘌呤，由于稳定RNA积累速率暂时加快，MS核苷酸浓度会迅速下降。然而，再过5分钟后，RNA积累停止，尽管MS核苷酸浓度仍然很低。此外，如果将甘氨酸或甲硫氨酸添加到补充有嘌呤的甲氧苄啶抑制的培养物中，RNA积累不会恢复，尽管MS核苷酸浓度迅速下降。当两种氨基酸都存在时，MS核苷酸浓度下降且稳定RNA合成恢复。这些发现表明，甲氧苄啶抑制的细菌中MSI核苷酸浓度不是控制稳定RNA合成的唯一因素。有可能在RC(str)细胞含有高浓度MS核苷酸的时期，一些对MSI介导的稳定RNA合成控制很重要的因素被不可逆地灭活了。然而，由于抗生素（如氯霉素）在相同条件下既能消除高MS核苷酸浓度又能使稳定RNA积累迅速恢复，更有可能是一种额外的控制机制发挥了作用。
