Mapping of VSG genes on large expression-site chromosomes of Trypanosoma brucei separated by pulsed-field gradient electrophoresis.

We have modified the conditions for pulsed-field gradient electrophoresis of Trypanosoma brucei (strain 427) to resolve large chromosomes, which previously comigrated, into five distinct chromosome bands. Mapping of four different variant surface glycoprotein (VSG) genes, which were duplicatively activated in eight independent VSG gene switching events during later stages of infection, has shown that the activated gene is invariably translocated to the same chromosome band. Also, we have examined the chromosomal location of two VSG genes which were activated without duplication during early infection. The location of each is constant in its active and inactive state. These data identify a single chromosome band which is preferentially used for VSG gene expression during chronic infections in this trypanosome strain and support the concept that the transcription and the translocation of VSG genes are independently regulated.