(a) The 5’ ETS recruits UtpA (blue), UtpB (light red), U3 snoRNP (pink, RNA red) and additional factors such as Utp7 (green) and Sof1 (blue). Parts of Utp7 and Sof1 are flexible in the absence of the 18S rRNA and UtpB adopts a retracted conformation as shown by the presented cryo-EM structure of the 5’ ETS RNP. (b–d) The 5’ ETS RNP forms a separate module during the transcription and independent maturation of the 18S rRNA domains (5’, central, 3’major, 3’minor). Individual 18S rRNA domains can independently recruit assembly factors. The inferred structures of subsequent assembly intermediates (b–d) are based on the structure of the 5’ ETS RNP and individual rRNA domains and associated factors as seen in the fully assembled SSU processome (e, PDB 5WLC). Assembly factors transiently associated with a particular assembly stage are boxed and labeled (I, II, III). Factors in these boxes (I, II, III) together with U14, snR30 and others leave the maturing particle before SSU processome formation as indicated by an arrow in (d). (d) UtpB acts as a sensor for the 3’ end of the 18S rRNA by recognizing elements in the 3’ minor domain (H44, dark blue). A conformational change in UtpB and the recruitment of 3’ minor factors (dark-blue, boxed) lead to the formation of the SSU processome. (e) In the fully assembled SSU processome (PDB 5WLC) the 5’ ETS is cleaved at site A0. Pno1 is stabilized by the interaction with the rotated UtpB CTD tetramer and Utp14 occupies a binding site obstructed by a 5’ ETS RNA linker in the 5’ ETS RNP.