Abstract
To understand the coordination of gene expression in the Salmonella typhimurium ilvIH-leuO-leuABCD gene cluster, we had previously identified a 72-bp AT-rich (78% A+T) DNA sequence element, AT4, which was capable of silencing transcription in a promoter nonspecific manner. LeuO protein provided in trans relieved (derepressed) AT4-mediated gene silencing (transcriptional repression), but underlying mechanisms remained unclear. In the present communication, the 72-bp DNA sequence element is further dissected into two functional elements, AT7 and AT8. LeuO binds to the 25-bp AT7, which lies closest to the leuO promoter in the AT4 DNA. After deletion of the AT7 DNA sequence responsible for LeuO binding from AT4, the remaining 47-bp AT-rich (85% A+T) DNA sequence, termed AT8, retains the full bi-directional gene-silencing activity, which is no longer relieved by LeuO. LeuO-mediated transcriptional derepression is restored when the LeuO binding site, AT7, is placed within close proximity to the gene silencer AT8. As a pair of functionally coupled transcription elements, the presence of an equal copy number of AT7 and AT8 within proximity is important for the transcription control. The characterization provides clues for future elucidation of the molecular details whereby LeuO negates the gene-silencing activity.
Highlights
The suppression of the leu-500 mutation in Salmonella typhimurium topAϪ strains [1] has been a paradigmatic phenomenon for explaining the importance of the effect of DNA supercoiling on transcriptional initiation [2]
To understand the coordination of gene expression in the Salmonella typhimurium ilvIH-leuO-leuABCD gene cluster, we had previously identified a 72-bp AT-rich (78% A؉T) DNA sequence element, AT4, which was capable of silencing transcription in a promoter nonspecific manner
Transcription-driven DNA supercoiling is indispensable in these processes, but the supercoiling itself alone is insufficient for triggering the gene expression coordination, because deleting part of the DNA sequence from the 1.9-kbp intervening region abolished the promoter relay mechanism [3]
Summary
The suppression of the leu-500 mutation in Salmonella typhimurium topAϪ strains [1] has been a paradigmatic phenomenon for explaining the importance of the effect of DNA supercoiling on transcriptional initiation [2].
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