[Creepy] Plasmids and Antibiotic Resistance
If the title didn’t give it away, plasmids are creepy. For those of you who are confused by that statement let’s cover what a plasmid is exactly. Plasmids are very short circular pieces of DNA that are like little virus’. What makes the little buggers creepy is that they aren’t technically living, but possess all the qualities that would make you think otherwise.
Plasmids bestow certain abilities to the bacteria that they live in. Interestingly they can also keep other competing plasmids out of the cell, so again creepy. They can either attach themselves to the host DNA strand, or can just sit in circular form and wait to be replicated when the host cell divides.
Plasmids range from 1000 base pairs to 1,000,000 base pairs and evolve much faster than your average bacteria or virus. When staph became multiple antibiotic resistant, that wasn’t because the bacteria adapted to antibiotics. That little trick was something that had been bestowed by plasmids.
So while most scientists are trying to revive antibiotics to prevent a potential epidemic, there are a few who are fighting a different kind of war, the war against plasmids.
Now a team from Duke and the University of Sydney in Australia has solved the structure of a key protein that drives DNA copying in the plasmids that make staphylococcus bacteria antibiotic-resistant. Knowing how this protein works may help researchers find new ways to stop the plasmids from spreading antibiotic resistance in staph by preventing the plasmids from copying themselves.
“If plasmids can’t replicate, they go away,” said lead author Maria Schumacher, an associate professor of biochemistry in the Duke University School of Medicine. “This is a fantastic new target for antibiotics.”
Through several years of laborious [and I am sure tedious] structural biology to figure out the specific shapes of the molecules involved, the research team has mapped out the structure and function of a protein called RepA, which is crucial to the plasmids’ ability to copy its DNA and make a new plasmid.
RepA is a protein that sticks to the beginning of the plasmid’s DNA sequence and starts the copying process. “This protein is essential to everything,” Schumacher said. “If you don’t have it, the plasmid will quickly cease to exist.”
Plasmids also need a way to prevent themselves from making too many copies, which would strangle their bacterial host, a sort of crowd control inside the host cell. The researchers have found that RepA is crucial to that function as well.
RepA naturally sticks together in pairs. When a pair of RepA proteins bumps into another pair, as when the cell is starting to get crowded with plasmids, the two pairs of RepA preferentially stick to each other. They form a complex back-to-back, a sort of figure 8 shape, with both having their DNA-grabbing parts facing outward.
When RepA forms this four-part molecule, the plasmids are said to be ‘handcuffed,’ because two rings of DNA are captured with the locked-up and non-functional RepA complex in the middle.
Once it is handcuffed like this, the plasmid will no longer replicate, so the handcuffed metaphor is very accurate in this sense. Schumacher says this mechanism is apparently how RepA prevents the plasmids from overpopulating the bacterial cell.
Schumacher says RepA is ubiquitous in the plasmid world and doesn’t bear much resemblance to other proteins, or to human proteins, making it an attractive drug target. This means that the molecule could be a new site to attack with antibiotics, ideally eliminating the plasmids responsible for antibiotic resistance.
Didn’t sufficiently creep you out? You may want the full study then, which can be found —here!
Maria A. Schumacher, Nam K. Tonthat, Stephen M. Kwong, Naga babu Chinnam, Michael A. Liu, Ronald A. Skurray, & Neville Firth (2014). Mechanism of staphylococcal multiresistance plasmid replication origin assembly by the RepA protein Proceedings of the National Academy of Sciences : 10.1073/pnas.1406065111