The procyclin-associated genes of Trypanosoma brucei are not essential for cyclical transmission by tsetse

EP and GPEET procyclins are the major surface glycoproteins of Trypanosoma brucei in the midgut of tsetse flies (Glossina spp.). The procyclin genes are located at the beginning of polycistronic transcription units and are followed by at least one procyclin-associated gene (PAG). The EP/PAG1 locus on one copy of chromosome X begins with the three genes EP1, EP2 and PAG1; the end of this unit has not been characterized previously. The EP/PAG2 locus on the other copy of chromosome X contains the same procyclin genes followed by PAG2 and PAG4. Here we show that the EP/PAG1 locus in AnTat1.1 has to be extended by three more PAGs, which we named PAG5, PAG2* and PAG4. The EP/PAG2 locus most likely evolved from the EP/PAG1 locus by deletion of a fragment from within PAG1 to PAG2*. The procyclin loci on the two copies of chromosome VI are indistinguishable, and contain the genes GPEET, EP3, PAG3 and GRESAG2.1. The mRNA levels of PAG1, PAG2 and PAG3 are transiently increased during differentiation of bloodstream forms to procyclic forms. Unexpectedly, procyclic forms of a PAG knockout clone lacking all eight PAGs in the procyclin loci were transmissible by Glossina morsitans. Furthermore, the deletion mutant could still establish midgut infections when competing with a tagged clone with the full complement of PAGs. Cyclical transmission was also possible when tsetse flies were infected with bloodstream forms of the deletion mutant, demonstrating that the PAGs are not essential for the differentiation of bloodstream to procyclic forms in vivo.