Primary structure of Trypanosoma cruzi small-subunit ribosomal RNA coding region: comparison with other trypanosomatids.

A Trypanosoma cruzi small subunit ribosomal RNA gene was sequenced from genomic recombinant plasmid clones. The assigned coding region was 2319 bp, the longest SSU rRNA gene described to date. On the basis of comparisons with published sequences from Crithidia fasciculata, Trypanosoma brucei, and Leishmania donovani, we conclude that the extra nucleotides in the T. cruzi gene occur in highly variable regions of rRNA genes. A phylogenetic analysis was performed with the SSU rRNA sequences from these four trypanosomatids and from Euglena gracilis as an outgroup. The Leishmania and Crithidia sequences were remarkably similar to each other (not separable statistically). Given the standard errors associated with the branching order of the two Trypanosoma species and the Leishmania-Crithidia branch, the actual topology cannot be unequivocally determined using only the ribosomal sequences analyzed here.