An unknown endonuclease cleaves at site 2 (also called site 2b in mouse) in the internal transcribed spacer 1 (ITS1) between the 18S rRNA and the 5.8S rRNA in the 45S precursor rRNA (pre-rRNA) while the pre-rRNA is contained in a 90S particle containing ribosomal proteins and assembly factors (Sloan et al. 2013). (The 90S particle is believed to be produced by addition of further subunits to the complex containing the pre-RNA and the small subunit (SSU) processome (inferred from yeast in Grandi et al. 2002).) The products are a 30S pre-rRNA which contains the 18S rRNA and a 32S pre-rRNA containing the 5.8S rRNA and the 28S rRNA. The cleavage splits the 90S particle into a pre-40S particle and a pre-60S particle and is believed to occur while the 5' region of the 45S rRNA is bound by the SSU processome. BOP1 (a subunit of the PeBoW complex), RBM28, NOL12 and RRP1 (NOP52) also participate in the cleavage (Sloan et al. 2013, Yoshikawa et al. 2015). Exonucleases including XRN2 and EXOSC10 (RRP6) of the exosome complex then remove further nucleotides from the end of the ITS (Sloan et al. 2013).
Grandi, P, Rybin, V, Bassler, J, Petfalski, E, Strauss, D, Marzioch, M, Schäfer, T, Kuster, B, Tschochner, H, Tollervey, D, Gavin, AC, Hurt, E
Sloan, KE, Mattijssen, S, Lebaron, S, Tollervey, D, Pruijn, GJ, Watkins, NJ
Yoshikawa, H, Ishikawa, H, Izumikawa, K, Miura, Y, Hayano, T, Isobe, T, Simpson, RJ, Takahashi, N
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