GC-responsive elements

The mechanism by which GCs exert these effects involves binding of the steroid to intracellular receptors, interaction of the steroid-receptor complex with GC response elements on DNA, transcription of genes, and synthesis of specific proteins (see Chapter 16, section III.C.2.). In some cases, the specific proteins responsible for the GC effect are known (e.g., the induction of phosphoenolpyruvate car-boxykinase that stimulates gluconeogenesis). In other cases, the proteins responsible for the GC effect have not yet been identified. 

M. R. Olive, W. J. Peacock, E. S. Dennis (1991) The anaerobic responsive element contains two GC-rich sequences essential for binding a nuclear protein and hypoxic activation of the maize adhl promoter. Nucl. Acids Res., 19 7053-7960... 

The up-regulation of GCS subunits by oxidative stress is mediated by different transcription factors such as nuclear factor kappa B (NF-kB), Sp-1, activator protein-1 (AP-1), activator protein-2 (AP-2) and antioxidant response elements (ARE) or electrophile responsive elements (EpRE). 

TATA box is the first step in the assembly of the RNA polymerase II transcription complex. The frequency of transcription initiation is often affected by binding certain transcription factors to upstream elements such as the CAAT box and the GC box. The activity of many promoters is affected by enhancers, regulatory sequences that may occur thousands of base pairs upstream or downstream of the gene they affect. (In yeast these sequences are called upstream activator sequences, or UAS.) The effects of enhancers can be complex. For example, a single gene may be controlled by the combined activities of several enhancers. Hormone response elements (Section 16.4) often act as enhancers. 

Problems and Limitations Unfortunately, derivatization is sometimes less selective than expected (Quevauviller et al. 1996), and the detector response is also species-derivative specific, which must be considered in quantification. Some of the derivatives, however, have almost no detector response (Mota and Simaes-Gon-calves 1996). As the separation typically is carried out at elevated temperature, only thermally stable species can be handled by GC. Another problem arises when coupling GC to element-selective detectors, such as ICP-atomic emission spectrometry (ICP-AES) or ICP-MS. The transfer line must also be heated up to the plasma, either by a pre-heated sheath gas (Orellana-Velado... 

Madsen CS, Hershey JC, Hautmann MB, White SL, Owens GK (1997) Expression of the smooth muscle myosin heavy chain gene is regulated by a negative acting GC-rich element located between two positive-acting serum response factor-binding elements. J Biol Chem 272 6332-6340... 

For several years, the Winchester Engineering and Analytical Center (WEAC) of the U.S. Food and Drug Administration (FDA) has evaluated and applied a variety of GC/HPLC-DCP approaches for true element specific and selective detection. Element selective, by and large, suggests that the response is relatively unique for a certain/specific element, but that other interferents may also produce similar responses. Element specific, by and large, suggests that the response on that particular detector is unique for that particular element, whatever its species or form. 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

So after more than 20 years of searching for an answer, we have much better data on the nature of star-to-star variations in the abundances of the light elements in GCs, but still no definitive understanding of the physical mechanism(s) responsible, nor of why metal poor field halo stars do not show these phenomena. 

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