טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSlomovic Shimyn
SubjectThe Double-Edged Role of RNA Polyadenylation in Human
Cells; a Time to Live, a Time to Die
DepartmentDepartment of Biology
Supervisor Professor Gadi Schuster
Full Thesis textFull thesis text - English Version


Abstract

   Polyadenylation of RNA is a post-transcriptional modification which plays an important role in gene expression. Interestingly, polyadenylation has two, negating forms. One form contributes to RNA stability and the other form, transient polyadenylation, targets RNA to polyadenylation-assisted degradation. At the onset of our research, a barrier spanned the biological world, dividing systems which utilized the former function of polyadenylation, from those that employed the latter. Stable polyadenylation was then associated with genes encoded in the nuclear genomes of eukaryotes while the transient form, with prokaryotes and organelles.

      However, we questioned as to the integrity of the barrier mentioned above. Is this divide as real as the cell walls and membranes that define it or does it simply draw its strength from our lack of knowledge and not actually exist at all? Are stable and transient poly(A) really segregated, or do we just not know of any cases in which they coexist?

            We identified the human mitochondrion as one of the first exceptions to breach this conceptual barrier and various systems have since followed, including the nuclei of yeast and human cells. We showed that its encoded transcripts, long known to harbor stable  poly(A) tails at their mature 3' ends, akin to nucleus-encoded mRNA, can undergo transient, internal polyadenylation as well. Indeed, the presence of transient polyadenylation in the mitochondrion is consistent with its evolutionary prokaryote origin and elucidates the mechanism of RNA degradation in this organelle, which yet needs to be deciphered. This newly attained knowledge gave rise to many questions; among them: How does this system differentiate between these opposing forms of RNA polyadenylation which coexist in the same compartment and which enzymes are involved? To gain insight, we analyzed the involvement of the mitochondrial poly(A)-polymerase and polynucleotide-phosphorylase in this process.

            Once realizing that the two negating roles of RNA polyadenylation can exist within the same cellular compartment, we asked if transient poly(A) tails are present in the cytoplasm of human cells. Indeed, both biologically and bioinformatically we were able to detect transient adenylation. Moreover, using RNAi to knock down the major RNA degradation machinery, we demonstrated that these molecules are indeed degradation intermediates and found in the cytoplasm.

            Together, our results show that RNA polyadenylation plays a double-edged role: By contributing to RNA stability and function or assisting in its degradation, polyadenylation determines for cellular RNA if it is a time to live or a time to die.