A role for RAD51 and homologous recombination in Trypanosoma brucei antigenic variation

Antigenic variation is an immune evasion strategy used by African trypanoso mes, in which the parasites periodically switch the expression of VSG genes that encode their protective variant surface glycoprotein coat. Two main r outes exist for VSG switching: changing the transcriptional status between an active and an inactive copy of the site of VSG expression, called the bl oodstream VSG expression site, or recombination reactions that move silent VSGs or VSG copies into the actively transcribed expression site. Nothing i s known about the proteins that control and catalyze these switching reacti ons. This study describes the cloning of a trypanosome gene encoding RAD51, an enzyme involved in DNA break repair and genetic exchange, and analysis of the role of the enzyme in antigenic variation. Trypanosomes genetically inactivated in the RAD51 gene were shown to be viable, and had phenotypes c onsistent with lacking functional expression of an enzyme of homologous rec ombination. The mutants had an impaired ability to undergo VSG switching, a nd it appeared that both recombinational and transcriptional switching reac tions were down-regulated, indicating that RAD51 either catalyzes or regula tes antigenic variation. Switching events were still detectable, however, s o it appears that trypanosome factors other than RAD51 can also provide for antigenic variation.