Two modes of activation of a single surface antigen gene of Trypanosoma brucei.

Several genes for variant antigens in trypanosomes are activated by duplicative translocation to a telomeric expression site. A second--nonduplicative--mode of activation is restricted to telomeric antigen genes. We show here that the single telomeric gene for antigen 221 can be activated in both ways. We also show that gene 221 is split and that the 5' 35 nucleotide sequence, common to all surface antigen mRNAs, is not encoded within 8.5 kb upstream of the 221 coding region. No major rearrangements are observed within 55 kb upstream of the 221 coding region upon nonduplicative activation. Gene inactivation is usually accompanied by deletion of the gene and at least 8.5 kb upstream and may involve conversion by another telomere. These results are not readily explained by a single expression site model. The duplicative gene 221 activation differs from conventional duplicative activation in the extent of the transposed segment, which is larger and may include the entire segment between gene and telomere.