Rna

FIGURE 6.09 Termination polymerase of Messenger RNA

When the mRNA reaches the hairpin of the terminator it pauses;

when it reaches the AAAAA UUUUU

sequence it falls off the template strand along with the newly synthesized RNA.

nator in the mRNA. The Rho hexamer does not form a closed ring, but instead is split open and resembles a lock washer in structure. The RNA sequence for Rho binding is poorly defined but is high in C and low in G. Rho can only bind to the growing mRNA chain once the RNA polymerase has synthesized the C-rich/G-poor recognition region and moved on. Rho moves along the RNA transcript and catches up with the RNA polymerase at the terminator stem and loop structure where the RNA polymerase pauses (Fig. 6.10). Rho then unwinds the DNA/RNA helix in the transcription bubble and separates the two strands.

Housekeeping genes are switched on all the time.

Some genes need activator proteins to switch them on.

How Does the Cell Know Which Genes to Turn On?

Some genes, known as housekeeping genes, are switched on all the time; i.e., they are expressed "constitutively." In bacteria, these often have both their -10 and -35 region promoter sequences very close or identical to consensus. Consequently, they are always recognized by the sigma subunit of RNA polymerase and are expressed under all conditions. Other constitutive promoters are further from consensus and expressed less strongly. Nonetheless, if only relatively low amounts of the gene product are needed, this is acceptable.

Genes that are only needed under certain conditions sometimes have poor recognition sequences in the -10 and -35 regions of their promoters. In such cases, the promoter is not recognized by sigma unless another accessory protein is there to help (Fig. 6.11). These accessory proteins are known as gene activator proteins and are different for different genes. Each activator protein may stimulate the transcription of one or more genes. A group of genes that are all recognized by the same activator protein will be expressed together under similar conditions, even if the genes are at different places on the DNA. Higher organisms have many genes that are expressed differently activator protein Protein that switches a gene on constitutive gene Gene that is expressed all the time

What Activates the Activator? 141

FIGURE 6.10 Termination by Rho

Rho first binds to the growing messenger RNA. When the RNA polymerase pauses at the termination site, Rho catches up and untwists the newly formed mRNA strand from the DNA. Subsequently, the mRNA and RNA polymerase fall off the DNA and Rho detaches from the mRNA.

FIGURE 6.10 Termination by Rho

Rho first binds to the growing messenger RNA. When the RNA polymerase pauses at the termination site, Rho catches up and untwists the newly formed mRNA strand from the DNA. Subsequently, the mRNA and RNA polymerase fall off the DNA and Rho detaches from the mRNA.

in different tissues. As a result, eukaryotic genes are often controlled by multiple activator proteins, more specifically known as transcription factors (see below).

What Activates the Activator?

Long ago, the Greek philosopher Plato pondered the political version of this question: "Who will guard the guardians?" In living cells, especially in more complex higher transcription factor Protein that regulates gene expression by binding to DNA in the control region of the gene

Activator protein

DNA Gene to be transcribed

FIGURE 6.11 Gene Activator Proteins

The activator protein first binds to the promoter region of the gene. Once bound, the activator protein facilitates the binding of the RNA polymerase. Gene transcription then commences.

Gene to be transcribed

Gene to be transcribed

FIGURE 6.12 MalT Changes Shape upon Binding Maltose

The MalT protein has a binding site complementary in shape to the sugar maltose. In step 1, MalT binds to maltose, which causes MalT to change shape. In step 2, the new conformation of MalT protein allows it to bind to DNA at a specific sequence found only in certain promoters. The gene thus activated is involved in the metabolism of maltose.

Step 1

Activator proteins often change shape in response to small molecules. Only one conformation binds to DNA.

organisms, there may indeed be a series of regulators, each regulating the next. What is the initial event? The cell must respond to some outside influence or must be influenced by other internal processes. The regulation of gene expression will be considered in more detail in Chapters 9 and 10. This chapter will be limited to a discussion of the basic mechanisms needed for a promoter to be functional.

As a simple example of an activator, consider the use of maltose by Escherichia coli. Maltose is a sugar made originally from the starch in malt and many other sources. It can be used by E. coli to satisfy all of its needs for energy and organic material. An activator protein, MalT, detects maltose and binds to it (Fig. 6.12).This causes the MalT protein to change shape, exposing its DNA-binding site. The original "empty" form of MalT cannot bind to DNA. The active form (MalT + maltose) binds to a specific sequence of DNA found only in the promoter region of genes needed for growth on maltose. The presence of MalT helps RNA polymerase bind to the promoter and transcribe the genes. The small molecule, in this case maltose, which causes gene

Negative Regulation Results from the Action of Repressors 143

Empty Lacl in operator site

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