Comparative analysis of group I intron evolution in microbial eukaryotes.

摘要

Group I introns are RNA-enzymes (ribozymes), some of which may trace their origin to the primordial RNA world. The biochemical properties of a few group I introns have been characterized, but little is known about how they evolve in nature. This is the central aim of the work presented here.; The tRNA-Leu intron found in bacteria and plastids is the oldest known group I intron. This ribozyme provides the best case for studying the long-term evolution of a group I intron. The results presented in Chapter 2 suggest that the distribution of this intron in plastids is best explained by vertical transmission, with no intron losses in land plants or a subset of the Stramenopiles and frequent loss among green algae, as well as in the red algae and their secondary plastid derivatives. All tested land plant introns could not self-splice in vitro and presumably have become dependent on a host factor to facilitate in vivo excision. In all other lineages, these ribozymes could either self-splice or complete the first step of auto-catalysis.; In Chapter 3 the extent of horizontal transfer of group I introns in the large subunit rDNA gene of the lichen family Physciaceae was determined. These results show five cases of horizontal transfer between species into homologous sites but do not support transposition into ectopic sites. This is in contrast to previous work with small subunit group I introns where strong support was found for ectopic transpositions.; In Chapter 4 rDNA intron insertion and loss at the population level was studied. Despite extensive preliminary data suggesting intron presence/absence differences both between and within species, the results here show this not to be the case. Instead, widespread rDNA heterogeneity was discovered.; Finally in Chapter 5, the origin of an rDNA spliceosomal intron from a pre-existing group I intron is reported. The evidence for this is based primarily on sequence similarity, phylogenetic analysis, and distribution data. This result is intriguing because it is the first time a group I intron has been implicated in the origin of a spliceosomal intron.