Ulmasov T, Hagen G, Guilfoyle T J
Department of Biochemistry, University of Missouri, 117 Schweitzer Hall, Columbia, MO 65211, USA.
Proc Natl Acad Sci U S A. 1999 May 11;96(10):5844-9. doi: 10.1073/pnas.96.10.5844.
Auxin-response factors (ARFs) bind with specificity to TGTCTC auxin-response elements (AuxREs), which are found in promoters of primary/early auxin-response genes. Nine different ARFs have been analyzed for their capacity to activate or repress transcription in transient expression assays employing auxin-responsive GUS reporter genes. One ARF appears to act as a repressor. Four ARFs function as activators and contain glutamine-rich activation domains. To achieve transcriptional activation on TGTCTC AuxREs in transient expression assays, ARFs require a conserved dimerization domain found in both ARF and Aux/IAA proteins, but they do not absolutely require their DNA-binding domains. Our results suggest that ARFs can activate or repress transcription by binding to AuxREs directly and that selected ARFs, when overexpressed, may potentiate activation further by associating with an endogenous transcription factor(s) (e.g., an ARF) that is bound to AuxREs. Transfection experiments suggest that TGTCTC AuxREs are occupied regardless of the auxin status in cells and that these occupied AuxREs are activated when exogenous auxin is applied to cells or when ARF activators are overexpressed. The results provide new insight into mechanisms involved with auxin regulation of primary/early-response genes.
生长素响应因子（ARFs）特异性地与TGTCTC生长素响应元件（AuxREs）结合，这些元件存在于初级/早期生长素响应基因的启动子中。在使用生长素响应性GUS报告基因的瞬时表达分析中，已对9种不同的ARFs激活或抑制转录的能力进行了分析。一种ARF似乎起阻遏作用。四种ARFs作为激活因子起作用，并含有富含谷氨酰胺的激活结构域。为了在瞬时表达分析中实现对TGTCTC AuxREs的转录激活，ARFs需要在ARF和Aux/IAA蛋白中都存在的保守二聚化结构域，但它们并非绝对需要其DNA结合结构域。我们的结果表明，ARFs可以通过直接与AuxREs结合来激活或抑制转录，并且某些ARFs在过表达时，可能通过与结合在AuxREs上的内源性转录因子（例如一种ARF）结合，进一步增强激活作用。转染实验表明，无论细胞中的生长素状态如何，TGTCTC AuxREs都被占据，并且当向细胞施加外源生长素或ARF激活因子过表达时，这些被占据的AuxREs会被激活。这些结果为生长素调节初级/早期响应基因的机制提供了新的见解。
