The Arabidopsis core exosome (Exo9) has a phosphorolytic activity due to the RRP41 subunit. The goal of this experiment was to determine the role of this intrinsic activity of Exo9 on the degradation of rRNA maturation by-products in Arabidopsis. Overall design: We complemented the rrp41 null mutant by expressing either a wild type RRP41 transgene (RRP41WT) or transgenes encoding catalytic inactive versions (RRP41Pi- and RRP41Pi-Cat-). High density mapping of intermediates of degradation of the 5' External Transcribed Spacer (5'ETS), a rRNA maturation by-product, was performed using 3' RACE-seq. 3' RACE-seq is an IIlumina-based sequencing strategy designed to map 3' extremities (Sikorska et al. 2017). Mapping of 5' external transcribed spacer (ETS) intermediates was performed in rrp41 complemented lines and in several exosome mutants.