Gene Regulation In Prokaryotes Eukaryotes Differs In Important Respects

In addition to transcription, eukaryotic cells employ a variety of mechanisms to regulate gene expression (Table 39-4). The nuclear membrane of eukaryotic cells physically segregates gene transcription from translation, since ribosomes exist only in the cytoplasm. Many more steps, especially in RNA processing, are involved in the expression of eukaryotic genes than of prokaryotic genes, and these steps provide additional sites for regulatory influences that cannot exist in prokaryotes. These RNA processing steps in eukaryotes, described in detail in Chapter 37, include capping of the 5' ends of the primary transcripts, addition of a polyadenylate tail to the 3' ends of transcripts, and excision of intron regions to generate spliced exons in the mature mRNA molecule. To date, analyses of eukary-otic gene expression provide evidence that regulation occurs at the level of transcription, nuclear RNA processing, and mRNA stability. In addition, gene amplification and rearrangement influence gene expression.

Owing to the advent of recombinant DNA technology, much progress has been made in recent years in the understanding of eukaryotic gene expression. However, because most eukaryotic organisms contain so much more genetic information than do prokaryotes and because manipulation of their genes is so much more limited, molecular aspects of eukaryotic gene regulation are less well understood than the examples discussed earlier in this chapter. This section briefly describes a few different types of eukaryotic gene regulation.

GAL4 GAL4

Active

A UAS

GAL1gene

Inactive

GAL1gene

LexA-GAL4

Active lexA operator

GAL1gene

Figure 39-16. Domain-swap experiments demonstrate the independent nature of DNA binding and transcription activation domains. The GAL1 gene promoter contains an upstream activating sequence (UAS) or enhancer that binds the regulatory protein GAL4 (A). This interaction results in a stimulation of GAL1 gene transcription. A chimeric protein, in which the amino terminal DNA binding domain of GAL4 is removed and replaced with the DNA binding region of the Ecoli protein LexA, fails to stimulate GAL1 transcription because the LexA domain cannot bind to the UAS (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).

Figure 39-17. Proteins that regulate transcription have several domains. This hypothetical transcription factor has a DNA-binding domain (DBD) that is distinct from a ligand-binding domain (LBD) and several activation domains (ADs) (1-4). Other proteins may lack the DBD or LBD and all may have variable numbers of domains that contact other proteins, including co-regulators and those of the basal transcription complex (see also Chapters 42 and 43).

Figure 39-17. Proteins that regulate transcription have several domains. This hypothetical transcription factor has a DNA-binding domain (DBD) that is distinct from a ligand-binding domain (LBD) and several activation domains (ADs) (1-4). Other proteins may lack the DBD or LBD and all may have variable numbers of domains that contact other proteins, including co-regulators and those of the basal transcription complex (see also Chapters 42 and 43).

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