Transcriptional regulatory proteins in eukaryotic cells can influence the initiation of transcription by affecting the stability or assembly of the basal transcription apparatus. Some regulatory proteins are activators and stimulate transcription; others are repressors and inhibit transcription.

Enhancers and insulators Enhancers are capable of affecting transcription at distant promoters. For example, an enhancer that regulates the gene encoding the alpha chain of the T-cell receptor is located 69,000 bp down-

16.23 An insulator blocks the action of an enhancer on a promoter when the insulator lies between the enhancer and the promoter.

^ Enhancer I can stimulate translation of gene A but its effect on gene B is blocked by the insulator.

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