哺乳动物转录调控激活方式

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在这里，我们对哺乳动物转录调控的激活方式进行讨论。以下为示意图

哺乳动物转录调控激活方式

• A factor is tissue-specific because it is synthesized only in a particular type of Cell . This is typical of factors that regulate development, such as homeodomain proteins.

• The activity of a factor may be directly controlled by modification. H5TF is converted to the active form by phosphorylation. API (a heterodimer between the subunitslunandFos) is converted to the active form by phosphorylating the lun subunit.

• A factor is activated or inactivated by binding a ligand. The steroid receptors are prime examples. Ligand binding may influence the localization of the protein (causing transport from cytoplasm to nucleus), as well as determining its ABI lity to bind to DNA .

• Availability of a factor may vary; for example, the factor nf-kb (which activates immunoglobulin KGENE s in B lymphocytes) is present in many cell types. It is sequestered in the cytoplasm, however, by the inhibitory protein I-KB. In B lymphocytes, NF-KB is released from I-KB and moves to the nucleus, where it activates transcription.

• A dimeric factor may have alternative partners. One partner may cause it to be inactive; synthesis of the active partner may displace the inactive partner. Such situations may be amplified into 652 CHAPTER 25 Activating Transcription networks in which various alternative partners pair with one another, especially among the HLH proteins.

• The factor may be cleaved from an inactive precursor. One activator is produced as a protein bound to the nuclear envelope and end oplasmic reticulum. The absence of sterols (such as cholesterol) causes the cytosolic domain to be cleaved; it then translocates to the nucleus and provides the active form of the activator.

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