“…Three sites (P, P9, and A1 [P2]) exist in the 59 ETS, four (D, A2, A3, and B1) in ITS1, three (E, C2, and C1) in ITS2, and two in the 39 ETS (B2 and B0;Sáez-Vasquez et al, 2004a, 2004bZakrzewska-Placzek et al, 2010;Lange et al, 2011;Weis et al, 2015aWeis et al, , 2015bSikorski et al, 2015;Tomecki et al, 2017). Moreover, functional studies of ribosome biogenesis mutants have identified the series of rRNA intermediates that occur during pre-rRNA processing (Lange et al, 2008(Lange et al, , 2011Abbasi et al, 2010;Zakrzewska-Placzek et al, 2010;Ohbayashi et al, 2011;Kumakura et al, 2013;Missbach et al, 2013;Hang et al, 2014;Weis et al, 2014Weis et al, , 2015bSikorski et al, 2015;Zhu et al, 2016). The high abundance of the P-A3 intermediate, which is easily detected in vivo, defines the major ITS1-first pathway in Arabidopsis, in which ITS1 cleavage at A3 occurs before complete removal of the 59 ETS in the 35S(P) primary transcript (Zakrzewska-Placzek et al, 2010;Lange et al, 2011;Sikorski et al, 2015).…”