Group II intron dynamics in heterologous hosts.

摘要

Group II introns are ribozymes with an innate ability to self-splice. They are found predominantly in bacterial and bacterial-derived organellar genomes, but not in the nuclear genomes of eukaryotes. In bacteria, group II introns often behave as mobile retroelements, invading host DNA and exploiting its machinery to complete the retromobility process. The object of my studies is the group II intron found in the Lactococcus lactis relaxase gene. To determine the nature of the group II intron-host relationship, we performed a genetic screen and identified several host factors that affect group II intron retromobility in Escherichia coli, which provides a facile genetic system. These functions are involved in DNA replication, RNA processing, energy metabolism and global regulation. RNase E, which is a central component of the RNA degradosome, inhibits retromobility by directly affecting intron RNA stability, whereas the global regulators cAMP and ppGpp foster group II intron invasion by exerting their effect on the DNA target. It might be that RNaseE, like cAMP and ppGpp, gauge the physiological status of the cell and accordingly modulate group II intron retromobility, especially under cellular stress. Together, these findings suggest in bacteria there is communication between the cell and the retroelement in regulating group II intron mobility, which in turn stimulates genetic diversity. In contrast, the eukaryotic cell, outside of bacterial-derived organelles, is an unfavorable environment for a group II intron. In addressing the debate on the relationship between group II introns and spliceosomal introns, we expressed a bacterial group II intron from a nuclear gene. We demonstrated that the intron is efficiently spliced in the yeast, Saccharomyces cerevisiae, but that the pre-mRNAs are subject to nonsense-mediated decay, and spliced mRNAs are translationally repressed. The host restriction thus exerted on the mRNA could have favored group II intron loss or evolution into spliceosomal introns, in the nuclear genomes of eukaryotes.