Expression of whole and hybrid genes in Trypanosoma equiperdum antigenic variation

A rapid technique involving the S1 nuclease resistance of RNA:DNA duplexes has been used to screen four Trypanosoma equiperdum variant surface glycoprotein (VSG) genes for evidence of hybrid gene structure in their transcribed regions. The results suggest that VSGs appearing early in a chronic infection each have a complete co-linear basic copy (BC) of their expressed gene while VSGs appearing later in infection are particularly associated with BC genes which are recombined before being expressed. The intensities of the S1-protected bands from hybrid VSGs indicate that the basic and expression linked copies are present in equivalent gene dosages. In addition, studies are presented on the expression of two additional VSG genes in T. equiperdum, VSG 4 and VSG 78, which (i) show that the basic copies are not located on telomeres even though one (VSG 4) is expressed early in infection and (ii) emphasize the role of a predominant expression site in T. equiperdum while nevertheless confirming the presence of multiple expression sites.