Control and function of the bloodstream variant surface glycoprotein expression sites in Trypanosoma brucei

African trypanosomes escape the host immune response through a periodical c hange of their surface coat made of one major type of protein, the variant surface glycoprotein. From a repertoire of a thousand variant surface glyco protein genes available, only one is expressed at a time, and this takes pl ace in a specialised expression site itself selected from a collection of a n estimated 20-30 sites. As the specialised expression sites are long polyc istronic transcription units, the variant surface glycoprotein is co-transc ribed with several other genes termed expression site-associated genes. How do the trypanosomes only use a single specialised expression site at a tim e? Why are there two dozen specialised expression sites? What are the funct ions of the other genes of these transcription units? We review the current ly available answers to these questions. (C) 2001 Australian Society for Pa rasitology Inc. Published by Elsevier Science Ltd. All rights reserved.