Heterologous Expression

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will talk about the importance of heterologous gene expression. We will talk about how it is used in science and technology, and also how we achieve heterologous gene expression.

Why is Technology Important in Biology?

How do you feed the 7.4 billion people on planet Earth, save your morning orange juice, and simultaneously manage to make a handful of these people very angry? Through agricultural technology!

Economist Thomas Malthus first came up with the concept of overpopulation - in 1798! Malthus suggested that the human population would continue to increase until there was no longer enough food to feed everyone, and famine and chaos would ensue. This is known as a Malthusian catastrophe.

Since then, many writers, scientists, and economists have predicted Malthusian catastrophe after Malthusian catastrophe. Yet a global-scale Malthusian catastrophe hasn't happened. Why not? Because, while the population of the world keeps increasing, so does agricultural technology. Agricultural production at the end of the 20th century was about 3.3 times higher in the U.S. than at the beginning of the 20th century, even though we have less farmland and fewer people working on farms.

Some of these innovations involve improving equipment such as tractors, or simply improving our knowledge of farming. Some of these innovations involve genetic technology, and heterologous gene expression.

Heterologous Expression

Heterologous expression describes the state where a gene is expressed in a different species than the one in which the gene originated. If you expressed a fish gene in a tomato, you would have heterologous expression, but beware! Despite what some conspiracy-oriented websites might tell you, there are no fish-tomatoes in your grocery store, or anywhere. There are a lot of angry conspiracy theorists with a lot to say about heterologous gene expression. We can't talk about all the conspiracy theories here, but remember to always look for reliable sources of information!

What does heterologous expression have to do with agriculture? Well, sometimes the gene you need to produce the crop you want doesn't exist in native varieties of that crop. For instance, you might want to produce corn that resists insects or cotton that resists herbicides, but you can't find a corn gene or a cotton gene that will do what you want. That's when heterologous expression comes in handy.

Heterologous Expression You Can Use

Here's an example. In the 1980s and 1990s, a virus called papaya ringspot virus was devastating papaya crops in Hawaii. Farmers were abandoning fields that were decimated by the virus. University scientists managed to introduce a heterologous gene from the virus itself into a papaya cultivar. This gene comes from the coat protein, or capsid, of the virus. It doesn't actually make the papaya trees sick, but it does make the papaya trees resistant to the virus. Thanks to this heterologous gene, papaya farmers rebounded, and the Hawaiian papaya market is still good.

Right now, a disease called huanglongbing is similarly devastating citrus trees. Citrus growers are really scared, and you should be too if you like orange juice! Huanglongbing starts with yellowed veins and discolored splotches on the leaves of citrus trees, but in time it will kill the whole tree. Scientists are currently investigating a gene called a defensin that comes from spinach as a way to save the citrus trees. Defensins provide antimicrobial activity, but they have been tested and shown to be non-toxic and hypoallergenic in humans.

Agricultural scientists are not the only people interested in heterologous gene expression. Developmental biologists also love heterologous expression, for completely different reasons. Sometimes it's hard to remove a gene from the context it has in your genome. Having another organism, such as the bacterium Escherichia coli, express your favorite gene can allow you to examine it outside of contexts that you don't 100% understand or control.

How It's Done

One way to make heterologous gene expression is to use a gene gun. A gene gun shoots tiny gold particles covered with your gene of interest. If the gold particle lands in the nucleus of the cell, the genes will separate from the gold particle and may incorporate themselves into the organism's genome.

Gene gun
Gene gun

Another way to make heterologous gene expression is with a virus. Basically, you cut up the gene of interest and insert it into the capsid of a virus. You then let your phony virus infect your organism of interest.

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