The fatal infection begins with the bite of a tsetse fly. The trypanosome parasite enters the bloodstream and starts to divide about every eight hours. The parasites stay in the bloodstream for months or even years, causing bouts of fever and headaches; sooner or later they invade the central nervous system, leading to coma and death. The single-celled protozoans survive for so long in the bloodstream by regularly acquiring new identities, thereby avoiding recognition and total elimination by the immune system. Surprisingly, the ability of the parasite to disguise itself resides in a transcriptional body discovered in the protozoan's nucleus.

The parasite shifts its coat by altering a protein called variant surface glycoprotein (VSG). It can express this type of protein from 1,000 different genes scattered across 20 sites on different chromosomes. Miguel Navarro, now at the CSIC Institute of Parasitology and Biomedicine in Granada, Spain, and Keith Gull of the University of Manchester in England conclude that the newly discovered transcriptional body reads one of these genes, much as the laser of a CD player plays back one track of a disc. Each gene represents a VSG alternative--a different melody--for the protozoan to express. "We believe a unique expression body does the job, attaching to any of these chromosome sites and transcribing only one VSG type at any time," Navarro explains. This body, which the team was able to localize in the nucleus by using fluorescent protein markers, was detected only when the parasite was in the bloodstream.

This article was originally published with the title Face Shift.

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