The use of incomplete genes for the construction of a Trypanosoma equiperdum variant surface glycoprotein gene.

The expression of Trypanosoma equiperdum variant surface protein (VSG) 78 is accomplished by the duplicative transposition of silent basic copy (BC) genes into a telomer-linked expression site to form an expression-linked copy (ELC). In two independent isolates expressing VSG 78, the ELC is a composite gene. The analysis of VSG 78 cDNA clones from these two Bo Tat 78 isolates and the respective BC genes revealed that both ELCs were constructed from the same three BC genes, a 3' BC which donated the last 255 bp of each ELC and two closely related 5' BCs. Although sequences of both 5' BC genes were found in each ELC, the junction with the 3' BC was provided by the same 5' BC in both cases. This 5' BC is an incomplete gene with insufficient open reading frame to code for a complete VSG and thus can only be used when joined to a competent 3' end. Furthermore, both 5' BC genes lack a conserved 14 nucleotide sequence found on all VSG mRNAs. These results support a model in which composite gene formation plays a role in the determination of the order of VSG expression. They also illustrate similarities between immunoglobulin gene and VSG gene construction.