Control of variant surface glycoprotein gene-expression sites in Trypanosoma brucei

Trypanosoma brucei has 20 similar telomeric-expression sites for variant su rface glycoprotein genes. Expression sites appear to be controlled at the l evel of transcription initiation, resulting in only one site being active a t any time. Snitching between expression sites occurs at a low rate. To ana lyse the switching mechanism, we used trypanosomes with two expression site s tagged with two different drug-resistance genes and selected these on aga rose plates containing both drugs. Double-resistant clones arose at a low f requency of 10(-7) per cell, but these behaved as if they rapidly switched between the two tagged expression sites and lost double resistance in the a bsence of selection. Using in situ hybridization we found that only 10% of the double-resistant cells had tno fluorescent spots corresponding to trans cribed expression sites. Our results suggest that: (i) a double expressor i s not a stable intermediate in expression site snitching; (ii) expression s ites are not independently switched on and off; and (iii) expression sites can be in a 'pre-active' silent state from which they can be readily activa ted.