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Valentin BEAUVAIS, Kévin MOREAU, Aurélia LE DANTEC, A. Rachid RAHMOUNI (CNRS CBM avenue de la recherche scientifique 45071 Orléans Cedex 2)
mRNA degradation deregulation is a key factor in the onset of diseases from cancers to neuropathologies. This mechanism is linked to the correct maturation of mRNA with numerous proteins, forming a mRNP (messenger ribonucleoprotein). In yeast, aberrant mRNP degradation is usually handled by the exosome complex, with Rrp6 as one of its catalytic subunit. By inducing the bacterial termination factor Rho in yeast, we increased the number of aberrant mRNP generated thus allowing us to study mRNP degradation mechanics. By deleting Rrp47, a key cofactor for Rrp6 activity, we were able to identify exosome targets but we also highlighted a population of transcripts that are insensitive to Rrp6 activity. We used a temperature inactivated Rat1 mutant to identify a co-existing degradation pathway mediated by Rat1, a 5’-3’ exonuclease able to target uncapped mRNA. We showed that transcripts vulnerable to Rat1 exhibit poorly capped mRNA during transcription initiation. Furthermore, we discovered that Rat1 dependent transcripts showed less pausing during transcription initiation and were transcribed by PolII having a poorly phosphorylated Ser5 CTD, 2 elements required for capping.