TARGETING OF EXOGENOUS DNA INTO TRYPANOSOMA-BRUCEI REQUIRES A HIGH-DEGREE OF HOMOLOGY BETWEEN DONOR AND TARGET DNA

Integration of exogenous DNA into the trypanosome genome occurs by hom ologous recombination only. To test whether a high degree of homology between donor and target DNA is required, we have inserted marker gene s for drug resistance into the promoter area of variant surface glycop rotein (VSG) gene expression sites of Trypanosoma brucei, using target ing fragments from two expression sites that are 92% identical. We obs erved integrations into expression sites that are known to be perfectl y matched to the donor flanks, and into subsets of uncharacterized exp ression sites that are specific for each type of targeting fragment, a nd that could be similar or identical to the donor flanks. This requir ement for very high homology was found in both procyclic and bloodstre am-form trypanosomes. We speculate that trypanosomes have a mismatch r epair system that suppresses recombination between divergent DNA seque nces, and we discuss ways in which the trypanosome might circumvent th e requirement for perfect DNA homology in the duplicative transpositio n of a VSG gene into a VSG gene expression site.