At least two transposed sequences are associated in the expression site of a surface antigen gene in different trypanosome clones.

The expression of several trypanosome surface antigen genes proceeds by duplication of a basic copy (BC) of the gene and transposition of the expression-linked copy (ELC) into an expression site. This site, which seems to be the same for different genes of the same repertoire, is located near a chromosome end. In the AnTat 1.1 antigen gene expression site, the ELC is found associated with another sequence that we have called the "companion." We found that this companion is the transposed copy of another sequence also located in an unstable DNA terminus, and that it is conserved in the expression site of AnTat 1.10 and AnTat 1.1B, two clones successively derived from AnTat 1.1. The companion sequence is not part of the surface antigen gene, but we may infer from extensive homologies with another ELC sequence (IoTat 1.3, J. E. Donelson, personal communication) that it represents a 5' residual fragment of a former ELC. In three other AnTat 1.1-like clones, the companion sequence was not found associated with the ELC. It is concluded that the expression-linked duplicative transposition of variable antigen genes is a flexible mechanism, which can apply to variably sized stretches of the same BC.