Studies on the evolution of the spliceosome and origin of the microsporidia.

摘要

The spliceosome is a large ribonucleoprotein complex composed of five small nuclear RNAs and over fifty proteins and is responsible for the removal of introns from pre-messenger RNA. Spliceosomal introns are prevalent among "crown" eukaryotes, whereas none has been found in those lineages thought to diverge earliest on the eukaryotic tree. However, the number of protein-coding genes sequenced from early-diverging eukaryotes is relatively low, creating a very small sample size. Since searching for introns directly in these organisms would be a nearly impossible task, I instead searched for evidence of spliceosomal components whose presence would be necessary for intron removal. The presence and conservation of such components could indicate that they are part of functional spliceosomes mediating the removal of spliceosomal introns.The parabasalid Trichomonas vaginalis was found to possess a highly conserved homolog of the spliceosomal protein PRP8. Essential in all organisms known to splice, PRP8 interacts abundantly with the pre-mRNA substrate and is likely at the catalytic centre of the spliceosome. The T. vaginalis homolog is highly conserved in regions of functional significance, indicating that spliceosomes could be present in trichomonads. Likewise, the microsporidian Nosema locustae also possesses spliceosomal components. Two highly conserved genes for U6 and U2 snRNAs were found, the genes are expressed and the two RNAs have the potential to interact with each other in the functional pairings characterized in organisms known to splice. The microsporidian snRNAs are also predicted to be part of a functional spliceosome, and introns are predicted to be present.During the course of this work, the ancient origin of the microsporidia has been called into question. Since the origin of the microsporidia bears on the interpretation of their possession of spliceosomal machinery, the phylogenetic position of the microsporidia was addressed by sequencing genes for TATA box binding protein (TBP), triosephosphate isomerase (TPI) and the second-largest subunit of RNA polymerase II (RPB2) from N. locustae. Taken as a whole, phylogenetic analyses with these sequences favour a much later origin for the microsporidia, and indicate a relationship between the microsporidia and the fungi. In addition, all genes are intron-lacking. A fungal origin for the microsporidia, coupled with the presence of old intron positions in TBP and TPI genes, suggests that spliceosomal introns have been lost throughout the course of microsporidian evolution.