Structure and evolution of the small subunit ribosomal RNA gene of Crithidia fasciculata

We present the cloning and sequence analysis of the nuclear-encoded Crithidia fasciculata small subunit (SSU) rRNA gene, the longest (2,206 bp) such gene yet characterized by direct sequence analysis. Much of the sequence can be folded to fit a phylogenetically conserved secondary structure model, with the additional length of this gene being accommodated within discrete variable domains that are present in eukaryotic SSU rRNAs. On the basis of sequence comparisons, we conclude that Crithidia contains the most highly diverged SSU rRNA described to date among the eukaryotes, and therefore represents one of the earliest branchings within the eukaryotic primary kingdom.