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Imagine that you love solving practical problems like how to build something with straws which will support some weight. No doubt your design will be better than those of your friends and/or siblings!

Well here is a challenge that is fun to think about but difficult to solve. Imagine that you are presented with 600 feet (19000 cm) of shoelace. You are also presented with some small sheets of plastic, each cut into an appropriate shape so that they form a hollow icosahedral container when assembled together. Your assignment is to get the shoe lace into the container!

So, how do we start? First maybe you crumple up the shoe lace to find how much volume it requires. Uh Oh! Bad news! The assembled container is actually considerably smaller than the shoelace requires. But on with the show! First perhaps we try to assemble the sides of the container around the shoelace. Guess what! It just doesn’t work. There is always lots of lace which can’t be crammed inside the container.

OK … on to plan B. We first assemble the container, and glue its sides together. There is a tiny hole at one end. Aha! Obviously we just thread the shoelace through the tiny hole into the container. Of course things don’t go as hoped. The lace bends, resisting being threaded, and soon it is impossible to shove any more into the container. Is this an exercise in futility? Mission impossible? Actually not!

There are extremely tiny structures called viruses which have been designed to solve all these problems. Most viruses are so tiny that they cannot be seen with a light (ordinary) microscope. However “small” does not mean poorly designed. A virus is a structure composed of a strand of genetic material or DNA (like the shoelace) and a protein coat (like the walls of the container). The virus’s genetic information manages to commandeer a living cell to form lots of virus genetic information (like lots of shoelaces), and also lots of containers. But now the task is to get the genetic information into the protein coats, all at the same time. We don’t need to worry however, those virus coats come assembled complete with a very powerful motor to shove the genetic material into the tiny containers.

Research carried out by Dr. Carlos Bustamente of California (and colleagues) has revealed how the packaging motor works with the Phi29 bacteriophage (virus) which exploits Bacillus subtilis cells. Bacillus is a bacterium which lives in soil and the human gut. Once the host Bacillus cell has formed lots of virus genetic material (DNA) and lots of icosahedral containers, a motor at the base of each container grabs the end of a DNA strand. The motor consists of a ring made up of 5 subunits. Four of these attach in sequence to the DNA molecule and one motor unit remains unattached, to coordinate the activity of the other four. Then the motor forces a small section of the DNA molecule into the container, meanwhile twisting the strand as it moves inward.

This process continues so forcefully that the DNA is compressed into a very tiny space, such that the DNA lies under 60 atmospheres of pressure! The motor itself is so strong that it exerts 15 – 20 times more intense a force than the strongest muscle. As to the 60 atmospheres of pressure, you may have seen a cork pop out of a carbonated beverage bottle. The carbon dioxide in that drink is only there at 5 – 6 atmospheres of pressure. Imagine if the beverage gases were compressed to 60 atmospheres! It would be downright dangerous!

The scientists involved in this project, declare that the packaging motor of the virus is surprisingly sophisticated in that the motor adjusts its speed and force as the container fills. By the end, the packaging speed has fallen by about 100 times. The whole system is extremely efficient in its use of chemical energy to produce mechanical force. Also, by the way, all five components of the ring motor must be functional, or the whole thing stops right away. That sounds a lot like irreducible complexity doesn’t it?

So, how interesting is it that something so tiny as a virus can exhibit so powerful a ring motor? Our technological society has not developed anything like such an elegant machine. But scientists hope to work on the idea soon! Once again we are vastly in debt for design ideas to the Creator of all things, including virus motors.

[For those with technical expertise in biology, look for a Youtube video called “Grabbing the Cat by the Tail: Discrete steps by a DNA packaging motor.”]

Moxie
July 2014

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