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16.2

Eukaryotic Gene Expression Is Regulated by Transcription Factors

For cell function in single-celled eukaryotes as well as the normal development of a multicellular organism from fertilized egg to adult, certain proteins must be made at just the right times and in just the right cells; these proteins must not be made at other times in other cells. Here are two examples from humans:

  1. In human pancreatic exocrine cells, the digestive enzyme procarboxypeptidase A makes up 7.6 percent of all the protein in the cell; in other cell types it is usually undetectable.

  2. In human breast duct cells, alpha-lactalbumin, a protein in breast milk, is made only late in pregnancy and during lactation. Alpha-lactalbumin is not made in any other cell types.

Clearly the expression of eukaryotic genes is regulated.

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  • The rate of transcription of a eukaryotic gene depends on the combination of transcription factors and other proteins binding to regulatory sequences associated with the gene.

  • DNA binding proteins have certain structural motifs in common that are important in their binding function.

  • Eukaryotic genes whose expression is coordinated share the same transcription factors.

As in prokaryotes, gene expression in eukaryotes can be regulated at several different points in the process of transcribing and translating the gene into a protein (Figure 16.7). In this section we will describe the mechanisms that result in the selective transcription of specific genes. The mechanisms for regulating gene expression in eukaryotes have similar themes as in those of prokaryotes. Both types of cells use DNA–protein interactions and negative and positive control. However, there are many differences, some of them dictated by the presence of a nucleus, which physically separates transcription and translation (Table 16.2).

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Figure 16.7 Potential Points for the Regulation of Gene Expression Gene expression can be regulated before transcription (1), during transcription (2, 3), after transcription but before translation (4, 5), at translation (6), or after translation (7).

Q: In prokaryotes, transcription and translation are often coupled in time and space. But in eukaryotes they are separated. What are the advantages of the nucleus as a compartment?

An advantage of the nucleus is compartmentation—the separation of transcription/processing and translation. This allows for finer regulation of gene expression. Also, the nucleus is protected from nucleases in the cytoplasm, which might hydrolyze mRNA and reduce its lifetime as it is made.

Activity 16.1 Eukaryotic Gene Expression Control Points

table 16.2 Transcription in Prokaryotes and Eukaryotes
Prokaryotes Eukaryotes
Locations of functionally related genes Often clustered in operons Often distant from one another with separate promoters
RNA polymerases One Three:

  1. transcribes rRNA

  2. transcribes mRNA

  3. transcribes tRNA and small RNAs
Promoters and other regulatory sequences Few Many
Initiation of transcription Binding of RNA polymerase to promoter Binding of many proteins, including RNA polymerase