Regulation of transcription, the initial step in all gene expression, is controlled by regulatory sequences of DNA, usually embedded in the noncoding regions of the genome.
REGULATORY SEQUENCES AND MOLECULES
Regulation of
transcription, the initial step in all gene expression, is controlled by
regulatory sequences of DNA, usually embedded in the noncoding regions of the
genome. The interaction between these DNA segments and regulatory molecules,
such as transcription factors, can engage or repress the transcriptional
machinery, influencing the kinds and amounts of products that are produced.
These DNA sequences flanking a gene are called cis-acting because they
influence expression of genes only on the same chromosome. A trans-acting factor
is the regulatory molecule itself, which can transit (diffuse) through the cell
from its site of synthesis to its DNA-binding site (Figure 32.2). For example,
a protein transcription factor (a trans-acting molecule) that regulates a gene
on chromosome 6 might itself have been produced from a gene on chromosome 11.
The binding of proteins to DNA is through structural motifs such as the zinc
finger (Figure 32.3), leucine zipper, or helix-turn-helix in the protein.
[Note: Some trans-acting factors can negatively affect gene expression.]
Figure 32.2 Cis-acting
elements and trans-acting molecules. mRNA = messenger RNA; Pol II = RNA
polymerase II.
Figure 32.3 Zinc (Zn) finger
is a common motif in proteins that bind DNA. Cys = cysteine; His = histidine.
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