One way to identify the expression pattern of a particular gene is to place a
reporter gene downstream of its promoter. In this configuration, the promoter gene will cause the reporter gene to be expressed only where and when the gene of interest is expressed. The expression distribution of the reporter gene can be determined by visualizing it. For example, the reporter gene
green fluorescent protein can be visualized by stimulating it with blue light and then using a
digital camera to record green
fluorescent emission. If the promoter of the gene of interest is unknown, there are several ways to identify its spatiotemporal distribution.
Immunohistochemistry involves preparing an
antibody with specific affinity for the protein associated with the gene of interest. This distribution of this antibody can then be visualized by a technique such as fluorescent labeling. Immunohistochemistry has the advantages of being methodologically feasible and relatively inexpensive. Its disadvantages include non-specificity of the antibody leading to
false positive identification of expression. Poor penetrance of the antibody into the target tissue can lead to
false negative results. Furthermore, since immunohistochemistry visualizes the protein generated by the gene, if the protein product diffuses between cells, or has a particularly short or long
half-life relative to the
mRNA that is used to
translate the protein, this can lead to distorted interpretation of which cells are expressing the
mRNA.
In situ hybridization is an alternate method in which a "probe," a synthetic
nucleic acid with a sequence
complementary to the mRNA of the gene, is added to the tissue. This probe is then chemically tagged so that it can be visualized later. This technique enables visualization specifically of mRNA-producing cells without any of the artifacts associated with immunohistochemistry. However, it is notoriously difficult, and requires knowledge of the
sequence of
DNA corresponding to the gene of interest. A method called
enhancer-trap screening reveals the diversity of spatiotemporal gene expression patterns possible in an organism. In this technique, DNA that encodes a reporter gene is randomly inserted into the genome. Depending on the gene
promoters proximal to the insertion point, the reporter gene will be expressed in particular tissues at particular points in development. While enhancer-trap derived expression patterns do not necessarily reflect the actual patterns of expression of specific genes, they reveal the variety of spatiotemporal patterns that are accessible to evolution. Reporter genes can be visualized in living organisms, but both immunohistochemistry and
in situ hybridization must be performed in
fixed tissues. Techniques that require fixation of tissue can only generate a single temporal time point per individual organism. However, using live animals instead of fixed tissue can be crucial in dynamically understanding expression patterns over an individual's lifespan. Either way, variation between individuals can confound the interpretation of temporal expression patterns. ==Methods to control spatiotemporal gene expression==