ABSTRACT
The protozoan parasite Trypanosoma brucei develops antigenic variation to escape the immune response of its host. To this end, the trypanosome genome contains multiple telomeric expression sites competent for transcription of variant surface glycoprotein genes, but as a rule only a single antigen is expressed at any time. We used reverse transcription-PCR (RT-PCR) to analyse transcription of different segments of the expression sites in different variant clones of two independent strains of T. brucei. The results indicated that RNA polymerase is installed and active at the beginning of many, if not all, expression sites simultaneously, but that a progressive arrest of RNA elongation occurs in all but one site. This defect is linked to inefficient RNA processing and RNA release from the nucleus. Therefore, functional transcription in the active site appears to depend on the selective recruitment of a RNA elongation/processing machinery.
Subject(s)
DNA-Directed RNA Polymerases/metabolism , Gene Expression Regulation , RNA, Protozoan/metabolism , Trypanosoma brucei brucei/genetics , Variant Surface Glycoproteins, Trypanosoma/genetics , Animals , Cell Nucleus/metabolism , Genetic Variation/genetics , Polymerase Chain Reaction , RNA Processing, Post-Transcriptional , RNA, Messenger/metabolism , Reverse Transcriptase Polymerase Chain Reaction , Sequence Analysis, DNA , Telomere/genetics , Transcription, Genetic , Trypanosoma brucei brucei/growth & development , Variant Surface Glycoproteins, Trypanosoma/metabolismABSTRACT
In Trypanosoma brucei, several genes termed ESAGs for expression site-associated genes are contained within the polycistronic transcription units of the VSG gene, and their transcription is coordinately regulated during the parasite life-cycle. Since the VSG mRNA is characterized by a drastic destabilization under conditions where translation is inhibited, we examined if this post-transcriptional control also applies to the ESAG mRNAs. While the ESAG 7/6 mRNA behaved like the VSG mRNA, the ESAG 8 and ESAG 3 mRNAs did not. We ascribe this differential behaviour to the residual transcription that still occurs only in the ESAG 7/6 region of the VSG unit under conditions where this unit is down-regulated.
