More than 170 proteins are necessary for assembly of ribosomes in eukaryotes. However, cofactors that function with each of these proteins, substrates on which they act, and the precise functions of assembly factors-e.g., recruiting other molecules into preribosomes or triggering structural rearrangements of pre-rRNPs-remain mostly unknown. Here we investigated the recruitment of two ribosomal proteins and 5S ribosomal RNA (rRNA) into nascent ribosomes. We identified a ribonucleoprotein neighborhood in preribosomes that contains two yeast ribosome assembly factors, Rpf2 and Rrs1, two ribosomal proteins, rpL5 and rpL11, and 5S rRNA. Interactions between each of these four proteins have been confirmed by binding assays in vitro. These molecules assemble into 90S preribosomal particles containing 35S rRNA precursor (pre-rRNA). Rpf2 and Rrs1 are required for recruiting rpL5, rpL11, and 5S rRNA into preribosomes. In the absence of association of these molecules with pre-rRNPs, processing of 27SB pre-rRNA is blocked. Consequently, the abortive 66S pre-rRNPs are prematurely released from the nucleolus to the nucleoplasm, and cannot be exported to the cytoplasm. In eukaryotes, 79 ribosomal proteins associate with ribosomal RNA (rRNA) to produce 40S and 60S ribosomal subunits (Woolford and Warner 1991). Three of the four rRNAs in mature ribosomes are derived from the 35S-45S rRNA precursor (pre-rRNA) transcribed by RNA polymerase I, while the fourth rRNA, 5S rRNA, is transcribed from separate genes by RNA polymerase III. The 35S-45S primary transcript is packaged into a 90S ribonucleoprotein particle (RNP), together with a subset of assembly factors and ribosomal proteins. Subsequent steps trigger folding, modification, and processing of prerRNAs and association of additional assembly factors and ribosomal proteins in 43S and 66S assembly intermediates. These pre-rRNPs undergo further maturation in the nucleolus, nucleoplasm, and then cytoplasm to form functional 40S and 60S ribosomal subunits, respectively ( Fig. 1A; FromontRacine et al. 2003;Raué 2003;Granneman and Baserga 2004). Preribosomal particles in the assembly pathway are distinguished by the presence of successive prerRNA processing intermediates (Fig. 1A). However, it is not clear into which of the consecutive preribosomes 5S rRNA and each ribosomal protein are incorporated, which assembly factors are required to recruit these molecules, or how they do so. Furthermore, the mechanisms by which constituents of nascent ribosomes facilitate folding, processing, and modification of pre-rRNAs remain elusive.5S rRNA is essential for maturation of preribosomes and for the function of mature ribosomes (Van Ryk et al. 1992;Dechampesme et al. 1999;Kiparisov et al. 2005). Steitz and coworkers defined a pathway of assembly of 5S rRNA into ribosomes in HeLa cells. Newly synthesized 5S pre-rRNA binds transiently to the La protein (Rinke and Steitz 1982;Yoo and Wolin 1994). Following 3Ј-end maturation, 5S rRNA binds to ribosomal protein rpL5, then assembles into ribosomes (...