The major components of the paraflagellar rod of Trypanosoma brucei are two similar, but distinct proteins which are encoded by two different gene loci.

The flagellum of trypanosomatids contains two major structural elements, the microtubular axoneme, which is the mechanochemical motor of the cell, and the paraflagellar rod (PFR), a highly organized, three-dimensional fiber network of unknown function. Its occurrence is limited to a few groups of unicellular eukaryotes, the Euglenoids, Dinoflagellates, and Kinetoplastids. The PFR of trypanosomes consists of two similar proteins of approximate molecular masses of 69 and 73 kDa. This laboratory and others have earlier characterized the genes coding for the 69-kDa PFR species in Trypanosoma brucei and Trypanosoma cruzi. The present study reports the characterization of the genes coding for the 73-kDa PFR protein from T. brucei. Their gene product, PFR-C, is closely related to, but clearly distinct from, the 69 kDa species PFR-A. This finding indicates that the PFR fibers of the flagellum might represent heteropolymers formed by PFR-A and PFR-C, akin to the situation seen in microtubules or intermediate filaments. PFR-A and PFR-C are each coded for by a similarly organized cluster of four closely spaced, tandemly arranged genes. The 5'-untranslated region of the first gene in each cluster is different from those of the following three genes, which are identical among each other. Conversely, the 3'-untranslated regions of the first three genes of each cluster are identical, while the corresponding region of the fourth gene is different. This unusual organization leads to the generation of mRNAs which contain identical coding sequences but different 5'- and 3'-noncoding regions.