|M.Sc Student||Goler-Baron Vicky|
|Subject||Transcriptions in The Nucleus and mRNA Decay in the|
Cytoplasm are Coupled Processes
|Department||Department of Medicine||Supervisor||PROF. Mordechai Choder|
|Full Thesis text|
Controlling appropriate levels of messenger RNAs (mRNAs) is vital for any living cell. mRNA synthesis in the nucleus by RNA polymerase II (Pol II) and mRNA decay by cytoplasmic machineries equally determine these levels. Yet, little is known about possible cross talk between the two compartments. In yeast, a major mRNA decay is the deadenylation-dependent pathway that occurs in the cytoplasm. The deadenylated mRNA can be degraded either by XRN1 exonuclease that acts in the 5' to 3' direction or by the exosome which degrades the mRNA from its 3'-end.
Pol II is composed of a ten subunit core and a dissociable heterodimer, Rpb4/7.
As part of this research we have found that each Rpb4 and Rpb7 are required for efficient deadenylation and post-deadenylation steps. Whereas, Rpb4 is specifically involved in decay of mRNAs encoding Protein Biosynthetic Factors (PBF) mRNAs, Rpb7 affects the decay of both PBF and non-PBF mRNAs thus, have more general role in mRNA decay. In addition, we found that Rpb7 is also involved in 3’ to 5’ pathway of mRNA degradation. Thus, Rpb7 might be involved in the linkage between the two major cytoplasmic mRNA decay pathways.
We assume that cross talk between transcription and the major mRNA decay pathways is crucial for the appropriate control of gene expression. Since, Rpb4/7 is a nucleo-cytoplasmic shuttling heterodimer that interacts with Pol II and is required for mRNA decay, we examined whether mRNA decay in the cytoplasm depends on association of Rpb4/7 with the core Pol II in the nucleus. We show that specific mutations in the core Pol II, which compromise with the capacity of Pol II to recruit Rpb4/7, also compromise mRNA deadenylation and post deadenylation steps, such as 3’ to 5’ decay by the exosome. We present various control experiments demonstrating that this flawed mRNA decay is indeed due to defective interaction of Rpb4/7 with Pol II. We also propose that Rpb4/7 interacts with the transcript during transcription for later proper execution of deadenylation and degradation. In this way, Pol II impacts mRNA decay via a shuttling factor, Rpb4/7, which couples between the nuclear and cytoplasmic processes that determine mRNA level.