The ribosomal RNA processing machinery is recruited to the nucleolar domain before RNA polymerase I during Xenopus laevis development

Transcription and splicing of messenger RNAs are temporally and spatially c oordinated through the recruitment by RNA polymerase II of processing facto rs. We questioned whether RNA polymerase I plays a role in the recruitment of the ribosomal RNA (rRNA) processing machinery. During Xenopus laevis emb ryogenesis, recruitment of the rRNA processing machinery to the nucleolar d omain occurs in two steps: two types of precursor structures called prenucl eolar bodies (PNBs) form independently throughout the nucleoplasm; and comp onents of PNBs I (fibrillarin, nucleolin, and the U3 and U8 small nucleolar RNAs) fuse to the nucleolar domain before components of PNBs II (B23/NO38) . This fusion process is independent of RNA polymerase I activity, as shown by actinomycin D treatment of embryos and by the lack of detectable RNA po lymerase I at ribosomal gene loci during fusion. Instead, this process is c oncomitant with the targeting of maternally derived pre-rRNAs to the nucleo lar domain. Absence of fusion was correlated with absence of these pre-rRNA s in nuclei where RNA polymerase II and III are inhibited. Therefore, durin g X. laevis embryogenesis, the recruitment of the rRNA processing machinery to the nucleolar domain could be dependent on the presence of pre-rRNAs, b ut is independent of either zygotic RNA polymerase I transcription or the p resence of RNA polymerase I itself.