Harpers

392 / CHAPTER 39GAL4 +1ActiveAUASGAL1 geneLexA–GAL4+1InactiveBUASGAL1 geneLexA–GAL4 +1ActiveClexAoperatorGAL1 geneFigure 39–16. Domain-swap experiments demonstrate the independent nature of DNA binding and transcriptionactivation domains. The GAL1 gene promoter contains an upstream activating sequence (UAS) or enhancer thatbinds the regulatory protein GAL4 (A). This interaction results in a stimulation of GAL1 gene transcription. A chimericprotein, in which the amino terminal DNA binding domain of GAL4 is removed and replaced with the DNA bindingregion of the E coli protein LexA, fails to stimulate GAL1 transcription because the LexA domain cannot bind to theUAS (B). The LexA–GAL4 fusion protein does increase GAL1 transcription when the lexA operator (its natural target)is inserted into the GAL1 promoter region (C).Eukaryotic Genes Can Be Amplifiedor Rearranged During Developmentor in Response to DrugsDuring early development of metazoans, there is anabrupt increase in the need for specific molecules suchas ribosomal RNA and messenger RNA molecules forproteins that make up such organs as the eggshell. Oneway to increase the rate at which such molecules can beformed is to increase the number of genes available fortranscription of these specific molecules. Among therepetitive DNA sequences are hundreds of copies of ribosomalRNA genes and tRNA genes. These genes preexistrepetitively in the genomic material of the gametesLigand-binding domainDNA-binding domain2134Activationdomains1–4Figure 39–17. Proteins that regulate transcriptionhave several domains. This hypothetical transcriptionfactor has a DNA-binding domain (DBD) that is distinctfrom a ligand-binding domain (LBD) and several activationdomains (ADs) (1–4). Other proteins may lack theDBD or LBD and all may have variable numbers ofdomains that contact other proteins, includingco-regulators and those of the basal transcriptioncomplex (see also Chapters 42 and 43).Table 39–4. Gene expression is regulated bytranscription and in numerous other ways ineukaryotic cells.• Gene amplification• Gene rearrangement• RNA processing• Alternate mRNA splicing• Transport of mRNA from nucleus to cytoplasm• Regulation of mRNA stability