The Theme of Evolution in Prokaryotes
A remarkable thing about how quickly bacteria grow is that it is possible to see evolution happen on a really fast time scale. Prokaryotes adapt to their environment by two main methods
1. Swapping DNA with other prokaryotes
2. Accumulating mutations in the DNA they already have
We’re going to talk about how antibiotic resistance can evolve by each of these methods, and then we’re going to discuss using evolution as a tool for biotechnology.
Antibiotic Resistance from Exchanging Genes
Tetracycline is an antibiotic. In case you’re wondering, it’s a bacteriostatic antibiotic that works by inhibiting protein synthesis. Tetracycline resistance typically develops by bacterial expression of proteins that transport the antibiotic outside of cells. Antibiotic resistance that depends upon an exporter typically depends upon sharing of genes between strains since evolving a whole new transporter is hard.
Antibiotic Resistance from Genomic Mutation
Streptomycin is antibiotic that, like tetracycline, inhibits protein synthesis. Streptomycin resistance, however, typically follows a different route. A shift in one amino acid in a ribosomal protein can completely protect bacteria from streptomycin activity. All cells, including bacterial cells, occasionally make slightly imperfect copies of their DNA which leads to shifts in proteins. Streptomycin resistance typically comes from a mutation in the gene that encodes this specific ribosomal protein.
Evolution and Biotechnology
Evolution and spread of antibiotic resistance is worrisome, and we will talk about that more in the Real Worldsection. Evolution in bacteria can be used to perform useful tasks, however. In a process called directed evolution, natural selection can be used to make microbes do new, useful things for us.
Directed evolution might work like this. Let’s say we had a bacterial strain that turned bright green whenever iron was around. Imagine this activity depended upon a protein called IroN, produced from the gene iroN. But maybe we don’t care about iron, maybe we're more interested in lead. And you’d be right to be worried about lead levels; lead is quite toxic to humans. So a bacterium that can indicate lead levels might be a very useful thing to have around.
If we wanted to make a bacterial test for lead levels, we could expose the bacteria to lead and just wait. Eventually a strain that turned green in response to lead might develop. But we might have to wait a really long time. One way you could help the bacteria evolve is to help them mutate. Brief exposure to X-rays or UV light or to various hazardous chemicals can lead to much higher mutation rates in DNA. This is a reasonable route, and is done in lots of situations.
We're in a better situation here, though. We already know that these bacteria produce a response to iron that depends in the iroN gene, which makes the IroN protein. We can get mutant copies of the iroN gene by PCRing the gene with a special DNA polymerase, called an error-prone polymerase, that makes a lot of mistakes while copying the gene.
After you have mutant versions of iroN , you can clone and insert in into a plasmid. This plasmid is transformed into bacterial cells just like we transformed bacteria with SUPER APE. All you need to do next is grow bacteria containing different iroN mutants and look for those that turn green in response to lead exposure.
We talked about testing for lead above, but you could look for a number of things. You could find bacteria that can grow in high levels of oil (to be used in cleaning up oil spills). Or maybe you work for a company that produces the amino acid cysteine, and you want to develop bacteria that make even more of it. Or, finally, maybe you want to develop bacteria that can "smell" chocolate cake. Umm, we’re not sure why, but you might.
We can try to evolve bacteria that can do any of these things. Selecting for bacteria that grow in high levels of oil might be more like looking for a streptomycin mutant. We could grow bacteria, perhaps after mutating their genomes broadly, in oil and look for those bacteria that survive.
If we wanted to find bacteria that could produce more cysteine, or those that can smell chocolate cake, you might want to start by mutating genes that make the proteins that produce cysteine or mutate genes that sense umm something that’s sort of similar to chocolate cake. Good luck with that last one. But we bet you could get the cysteine idea to work.
Check out this cartoon.