An expression-site-associated gene family of trypanosomes is expressed in vivo and shows homology to a variant surface glycoprotein gene.

Utilizing first-strand cDNA from different stages, a gene family was identified that is expressed in bloodstream form trypanosomes but not in cultured procyclic forms. This family of 50-100 genes, termed bloodstream-specific 1 (BS1), shares a chromosomal distribution pattern similar to the variant surface glycoprotein (VSG) genes and the expression-site-associated genes (ESAGs). The BS1 genes are expressed in several variants of Trypanosoma brucei brucei and in Trypanosoma brucei gambiense. Sequence analysis of five members of this gene family reveals the recently described ESAG 6 and ESAG 7 genes as well as the ESAG X gene to be members of this family. We have been unable to localize the BS1 gene product in the cell but show that chronically infected rabbit serum recognizes recombinant BS1 protein arguing for expression in vivo. Finally we note that the derived protein sequence for the BS1 genes suggests an evolutionary relationship with at least one variant surface glycoprotein gene, and hence these studies may provide clues to understanding the molecular origins of antigenic variation in trypanosomes.