The inactivation and reactivation of an expression-linked gene copy for a variant surface glycoprotein in Trypanosoma brucei.

We have previously shown that the gene for variant surface glycoprotein 118 of Trypanosoma brucei (strain 427) is activated by a duplicative transposition to a telomeric expression site. In chronically-infected animals, this expression-linked copy is lost when the 118 gene is replaced at the expression site by another variant surface glycoprotein gene. We show here that expression of the 118 gene can also be switched off without loss of the extra expression-linked copy. In two variants, called 1.8b and 1.8c, we find expression of the variant surface glycoprotein 1.8 gene, notwithstanding the continued presence of the 118 expression-linked copy. The 1.8 gene activated has a telomeric location, like the 118 expression-linked copy. In variant 1.8b, activation is accompanied by duplication of the 1.8 gene, resulting in an extra telomeric gene copy; in variant 1.8c it is not. Variants 1.8b and 1.8c both switch back preferentially to expression of the 118 gene. The 5'-flanking regions of the active, inactive and reactivated versions of the 118 expression-linked copy are indistinguishable by restriction mapping up to 28 kb. We conclude that there are at least two separate telomeric expression sites in our T. brucei strain. How these are switched on and off is unclear. The ability to retain expression-linked copies in inactive form may allow the trypanosome to re-programme the order in which variant surface glycoprotein genes are expressed.