A Similar Gene Is Shared by Both the Variant Surface Glycoprotein and Procyclin Gene Transcription Units of Trypanosoma brucei

The genes for the variant surface glycoprotein (VSG) and procyclin are expressed in a mutually exclusive manner during the life cycle of Trypanosoma brucei and synthesize the most abundant mRNAs specific to the bloodstream and procyclic stages of the parasite, respectively. Genes belonging to the polycistronic transcription unit of the VSG gene (expression site-associated genes [ESAGs]) are uniquely expressed in the bloodstream form, but some members of ESAG families (genes related to ESAGs [GRESAGs]) are independently transcribed outside the VSG gene expression site. We report here that a gene related to ESAG 2, GRESAG 2.1, is present and expressed in a procyclin gene transcription unit (PARP A locus), which is polycistronic. Members of the ESAG 2 family are thus present in the two major differentially stage-regulated transcription units of this parasite.