life11e_ch18

Gene expression can be modulated by DNA technology

DNA can be manipulated to be expressed in a different location or at a different rate. Researchers have learned a lot about gene expression and function by exploring questions such as these:

What happens when a gene is moved to a new location? A scientist may want to study a gene’s function and how it interacts with other genes by expressing the gene in a different organism—for example, by inserting a human gene into a bacterium, or a bacterial gene into a plant. Researchers have learned that expression of a transplanted gene requires a promoter and other regulatory sequences from the host organism: a bacterial promoter will not function in a plant cell, for example. The coding region of the gene of interest needs to be inserted between a promoter and a transcription termination sequence derived from the host organism, or from an organism that uses similar mechanisms for gene regulation.
What happens when the rate of expression of a gene is changed? Researchers have learned that a gene can be overexpressed in cells where it is normally expressed at lower levels, so that much more of its protein product is made. Overexpression is achieved by inserting a gene adjacent to a promoter that is very active in a cell, and then cloning the recombinant promoter–gene combination in a host cell.

Many thousands of experiments have shed light on the functions of genes and their protein products. One example involves a genetic system that probably evolved to prevent inbreeding in plants. Most plants produce flowers with both male and female parts, but many plant species are self-incompatible; they cannot self-pollinate (see Key Concepts 28.3 and 37.1). Their flowers produce a protein that recognizes “self” pollen, and prevents the pollen from fertilizing egg cells in the same flower (see Figure 37.4). Genetic crosses suggested that a particular multi-allelic gene, called the S gene, was responsible for self-incompatibility. Definitive proof was obtained when plants were transformed with recombinant DNA containing an S allele different from their own S alleles. The transgenic plants rejected not only their own pollen but also pollen from flowers that naturally carried the foreign S allele.