Control of Gene Expression in Prokaryotes
■ Gene expression in both prokaryotes and eukaryotes is regulated primarily by mechanisms that control the initiation of transcription.
■ Binding of the ct subunit in an RNA polymerase to a promoter region is the first step in the initiation of transcription in E. coli.
■ The nucleotide sequence of a promoter determines its strength, that is, how frequently different RNA polymerase molecules can bind and initiate transcription per minute.
■ Repressors are proteins that bind to operator sequences, which overlap or lie adjacent to promoters. Binding of a repressor to an operator inhibits transcription initiation.
■ The DNA-binding activity of most bacterial repressors is modulated by small effector molecules (inducers). This allows bacterial cells to regulate transcription of specific genes in response to changes in the concentration of various nutrients in the environment.
■ The lac operon and some other bacterial genes also are regulated by activator proteins that bind next to promoters and increase the rate of transcription initiation by RNA polymerase.
■ The major sigma factor in E. coli is ct70, but several other less abundant sigma factors are also found, each recognizing different consensus promoter sequences.
■ Transcription initiation by all E. coli RNA polymerases, except those containing ct54, can be regulated by repres-sors and activators that bind near the transcription start site (see Figure 4-16).
■ Genes transcribed by ct54-RNA polymerase are regulated by activators that bind to enhancers located =100 base pairs upstream from the start site. When the activator and ct54-RNA polymerase interact, the DNA between their binding sites forms a loop (see Figure 4-17).
■ In two-component regulatory systems, one protein acts as a sensor, monitoring the level of nutrients or other components in the environment. Under appropriate conditions, the y-phosphate of an ATP is transferred first to a histi-dine in the sensor protein and then to an aspartic acid in a second protein, the response regulator. The phosphory-lated response regulator then binds to DNA regulatory sequences, thereby stimulating or repressing transcription of specific genes (see Figure 4-18).
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