Getting tangled into knots is rarely a desirable situation. Yet there is a protein whose state of entanglement is very profitable since it is used to kill off bacteria which take up food and space. Microcin J25 is a small antibacterial peptide synthesized by another bacterium - Escherichia coli - when times are rough. To cut a long story short, the targeted bacteria die off leaving refreshment and room for their rivals.
In the 1990s, the structure of microcin J25 was thought to be cyclic. Only when it was synthesized in the lab, it proved to be inactive. So the hypothesis had to be rethought. In 2003, scientists suggested another structure which resembled a lasso whose tail folds over and enters the noose. This turned out to be highly stable and very difficult to denature. But how does it kill off bacteria?
For such a small entity to be able to destroy something many times its size, it must meddle with an essential biological pathway. Indeed it does. Microcin J25 interferes with protein synthesis. No less. It does this by taking up residence in the RNA polymerase - the enzyme which performs RNA elongation. In doing so, the RNA molecule can no longer grow. Consequently, proteins cannot be synthesized. And the bacteria die. It's cruel but effective.
Such a discovery is exciting for the design of antibacterial drugs. To date, there is one well-known inhibitor of bacterial RNA synthesis - Rifampicin - which binds to the RNA molecule that is in the process of being synthesized. Microcin J25 hunts down Gram-negative bacteria; however, it could be tinkered to provide variants which could decimate Gram-positive bacteria and even eukaryotes. All thanks to a knot.