| The occurrence of genome instability has been implicated in many detrimental outcomes, from cell death to cancer progression, as well as many other human diseases. Studies in yeast and mammalian cells have revealed that one cause of instability is the occurrence of RNA-DNA hybrids. Previously thought to be a rare consequence of transcription, in this study, I expand both the number of processes, as well as the number of mutants susceptible to RNA-DNA hybrid formation. I find that mutants defective in transcriptional repression, elongation, RNA export and RNA degradation exhibit increased hybrids and associated genome instability. I propose that even under wild-type conditions, hybrids readily form at many loci, likely due to transcriptional errors, but their accumulation is kept in check by two evolutionarily conserved RNase H enzymes. In this study, I also discuss one mechanism by which RNA-DNA hybrid formation can be mediated, via the strand-exchange protein Rad51p. In vivo and in vitro data indicate that Rad5lp facilitates the invasion of RNA into duplex DNA through an inverse strand exchange process; this Rad51-mediated hybrid formation may be a byproduct of the ability of Rad51 to form filaments on duplex DNA. This leads to a model in which RNA-DNA hybrids are kept under check by limiting Rad51 binding to undamaged chromatin, as well as free RNA in the nucleus. In summary, my work has established RNA-DNA hybrids as a potent source for changing genome structure and expanded the processes and mechanisms by which hybrids form, and can be removed. |
