SURVIVAL OF TRYPANOSOMA-BRUCEI IN THE TSETSE-FLY IS ENHANCED BY THE EXPRESSION OF SPECIFIC FORMS OF PROCYCLIN

African trypanosomes are not passively transmitted, but they undergo s everal rounds of differentiation and proliferation within their interm ediate host, the tsetse fly. At each stage, the survival and successfu l replication of the parasites improve their chances of continuing the life cycle, but little is known about specific molecules that contrib ute to these processes. Procyclins are the major surface glycoproteins of the insect forms of Trypanosoma brucei. Six genes encode proteins with extensive glutamic acid-proline dipeptide repeats (EP in the sing le-letter amino acid code), and two genes encode proteins with an inte rnal pentapeptide repeat (GPEET). To study the function of procyclins, we have generated mutants that have no EP genes and only one copy of GPEET. This last gene could not be replaced by EP procyclins, and coul d only be deleted once a second GPEET copy was introduced into another locus. The EP knockouts are morphologically indistinguishable from th e parental strain, but their ability to establish a heavy infection in the insect midgut is severely compromised; this phenotype can be reve rsed by the reintroduction of a single, highly expressed EP gene. Thes e results suggest that the two types of procyclin have different roles , and that the EP form, while not required in culture, is important fo r survival in the fly.