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Mol Cell Biol, March 1998, p. 1137-1146, Vol. 18, No. 3
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Activity of a Trypanosome Metacyclic Variant Surface Glycoprotein Gene Promoter Is Dependent upon Life Cycle Stage and Chromosomal Context

Sheila V. Graham,dagger Ben Wymer, and J. David Barry*

Wellcome Unit of Molecular Parasitology, The Anderson College, University of Glasgow, Glasgow G11 6NU, Scotland, United Kingdom

Received 7 July 1997/Returned for modification 11 August 1997/Accepted 1 December 1997

African trypanosomes evade the mammalian host immune response by antigenic variation, the continual switching of their variant surface glycoprotein (VSG) coat. VSG is first expressed at the metacyclic stage in the tsetse fly as a preadaptation to life in the mammalian bloodstream. In the metacyclic stage, a specific subset (<28; 1 to 2%) of VSG genes, located at the telomeres of the largest trypanosome chromosomes, are activated by a system very different from that used for bloodstream VSG genes. Previously we showed that a metacyclic VSG (M-VSG) gene promoter was subject to life cycle stage-specific control of transcription initiation, a situation unique in Kinetoplastida, where all other genes are regulated, at least partly, posttranscriptionally (S. V. Graham and J. D. Barry, Mol. Cell. Biol. 15:5945-5956, 1985). However, while nuclear run-on analysis had shown that the ILTat 1.22 M-VSG gene promoter was transcriptionally silent in bloodstream trypanosomes, it was highly active when tested in bloodstream-form transient transfection. Reasoning that chromosomal context may contribute to repression of M-VSG gene expression, here we have integrated the 1.22 promoter, linked to a chloramphenicol acetyltransferase (CAT) reporter gene, back into its endogenous telomere or into a chromosomal internal position, the nontranscribed spacer region of ribosomal DNA, in both bloodstream and procyclic trypanosomes. Northern blot analysis and CAT activity assays show that in the bloodstream, the promoter is transcriptionally inactive at the telomere but highly active at the chromosome-internal position. In contrast, it is inactive in both locations in procyclic trypanosomes. Both promoter sequence and chromosomal location are implicated in life cycle stage-specific transcriptional regulation of M-VSG gene expression.

* Corresponding author. Mailing address: Wellcome Unit of Molecular Parasitology, The Anderson College, University of Glasgow, 56 Dumbarton Road, Glasgow G11 6NU, Scotland, United Kingdom. Phone: 141 330 4875. Fax: 141 330 5422. E-mail: gbga05{at}udcf.gla.ac.uk.

dagger Present address: Institute of Virology, University of Glasgow, Glasgow G11 5JR, Scotland, United Kingdom.




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