When T7 attacks — watch a virus infect a cell

Illustration by Perry Shirley

By John Upton

The E. coli was doomed. This gut-dwelling microbe was trapped in the company of a predator. A T7 bacteriophage, a virus that propagates by inserting its DNA into bacteria, had been deposited nearby.

As the T7 landed on the surface of its prey, it was being watched by University of Texas Medical School researchers. They were watching the attack using cryo-electron tomography – a technique that creates 3-dimensional pictures from multiple microscopic images taken at freezing cold temperatures. They watched in graphic close-up detail as a virus infected a cell.

The virus was smaller than its prey. It looked like a bloated tick, with six folded legs made of protein at one end of its body. The legs formed a circle around a retracted tail. As the T7 landed on the bacterium, it extended some of its fibrous legs. It used the legs to walk along the surface of its prey, feeling for a suitable place to attack. Once it found the right location, it stopped walking and stood still. It planted its legs, extended its retracted tail and jammed it through the cell wall and into the victim. It pumped in its DNA. Then it retracted the tail, and the hole in the cell wall healed back over.

The attack was complete. The hapless bacterium now harbored the virus.

“The complete process we describe is unique to T7 and its relatives,” Ian Molineux, one of the researchers, told me. The study was published Thursday in the journal Science. “But some aspects, in particular fibers binding to the head and walking over the cell surface, are probably quite general.”