Jensen K F
University Institute of Biological Chemistry B, Copenhagen, Denmark.
Eur J Biochem. 1988 Aug 15;175(3):587-93. doi: 10.1111/j.1432-1033.1988.tb14232.x.
UTP-modulated attenuation of transcription is involved in regulating the synthesis of pyrimidine nucleotides in Escherichia coli. Thus, expression of two genes, pyrBI and pyrE, was shown to be under this type of control. The genes encode the two subunits of aspartate transcarbamylase and orotate phosphoribosyltransferase respectively. The levels of these enzymes are inversely correlated with the intracellular concentration of UTP. Modulation of attenuation seems to be a consequence of the effect of UTP concentration on the mRNA chain growth rate. Reducing the UTP pool retards RNA polymerase movement. Mechanistically this will couple the ribosomes translating a leader peptide gene more tightly to the elongating RNA polymerase. The ribosomes will then be more prone to prevent the folding of the mRNA chains into terminating hairpin structures when RNA polymerase is at the attenuator and has to decide whether transcription should terminate or continue into the structural genes. This paper described a study of pyrBI and pyrE gene regulation in cells where the ribosomes move slowly as a result of mutation in rpsL. It appears that expression of the two genes is hyper-regulated by the UTP pool in this type of cells. Furthermore, the attenuator model can only account for the results if it is assumed that UTP-concentration-dependent pausing of transcription occurs in vivo in the two pyr gene leaders such that RNA polymerase waits for the coupled ribosomes before transcribing into the attenuator regions.
UTP调节的转录衰减参与大肠杆菌嘧啶核苷酸合成的调控。因此，pyrBI和pyrE这两个基因的表达被证明受这种类型的控制。这两个基因分别编码天冬氨酸转氨甲酰酶和乳清酸磷酸核糖转移酶的两个亚基。这些酶的水平与UTP的细胞内浓度呈负相关。衰减的调节似乎是UTP浓度对mRNA链生长速率影响的结果。降低UTP库会阻碍RNA聚合酶的移动。从机制上讲，这会使翻译前导肽基因的核糖体与延伸的RNA聚合酶结合得更紧密。当RNA聚合酶处于衰减子位置并必须决定转录是应该终止还是继续进入结构基因时，核糖体将更倾向于阻止mRNA链折叠成终止发夹结构。本文描述了对rpsL突变导致核糖体移动缓慢的细胞中pyrBI和pyrE基因调控的研究。在这类细胞中，这两个基因的表达似乎受到UTP库的过度调控。此外，只有假设在体内两个pyr基因前导序列中发生UTP浓度依赖性的转录暂停，使得RNA聚合酶在转录进入衰减子区域之前等待耦合的核糖体，衰减子模型才能解释这些结果。
