Yeast Mediator facilitates transcription initiation at most promoters via a Tail-independent mechanism

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真核细胞基因转录抑制的分子机理

基因转录水平的调控是个复杂的过程,该方面的研究多集中于转录激活的机制上,但转录抑制也在基因表达中起重要作用．研究发现,核小体可抑制RNA聚合酶、转录因子与基因的结合,阻断转录起始．另外,基因转录抑制因子也可特异性地作用于转录过程．依作用机理,这些因子又可分为被动抑制因子和主动抑制因子两种．前者主要通过与激活因子竞争性结合基因的DNA结合位点或消弱激活因子与DNA结合的能力而减慢转录速率;后者通过与基因阻遏元件结合,直接抑制转录的起始．     

Sox regulates transcription of the sea urchin arylsulfatase gene

A 50 bp region from -194 bp to -144 bp of the arylsulfatase gene (HpArs) of the sea urchin, Hemicentrotus pulcherrimus, is related to the temporally regulated expression of this gene. This region contains a Sox (Sry-related HMG box)-binding site, and the introduction of sequence mutations to this site significantly reduced the activity of the HpArs promoter, even in the presence of the C15 enhancer, which consists of HpOtx and CAAT motifs. A protein that binds to the Sox-binding site in the 50 bp region of the HpArs gene was detected in nuclear extracts of mesenchyme blastulae and a protein synthesized in vitro using SoxB1 cDNA of another sea urchin, Strongylocentrotus purpuratus, also bound to this Sox site. These results suggest that HpSox, which is maternally expressed and remains abundant by the pluteus stage, is clearly implicated in regulation of the HpArs gene. The presence of a negatively acting cis element in this 50 bp region has also been detected.     

Lactones in the Flavor of Heated Pork Fat

Pork fat was heated at 160~170°C for 3 hr under bubbling with air, and the volatile compounds were collected in the cold trap. After the acidic compounds were removed from the volatile compounds by extraction with 3% aqueous sodium carbonate solution, lactones were obtained from the nonacidic compounds by saponification. Gas chromatographic analyses of lactones were carried out on the PEG-20M and Apiezon L packed columns, and then each lactone was fractionated by repeated gas chromatography. Each isolated lactone was identified by infrared spectrometry, and also three major lactones were identified by mass spectrometry. Consequently, γ-C₅—C₁₂ and δ-C₉, δ-C₁₀, δ-C₁₂ and δ-C₁₄ lactones were found in the flavor of heated pork fat. Gamma-lactones, especially γ-C₇, γ-C₈ and γ-C₉, were predominant in the flavor, and unsaturated lactones were not detected. Mechanisms for the formation of the lactones were discussed.