Identification of protein factors and U3 snoRNAs from a Brassica oleracea RNP complex involved in the processing of pre-rRNA.
In cruciferous plants, the primary pre-rRNA cleavage site (P site) is immediately downstream of four similar, highly conserved sequences (A(1), A(2), A(3) and B) located within the 5'-ETS (5'-external transcribed spacer). In the present study, we describe the characterization of a plant NF D (nuclear factor D) that binds and interacts specifically with this A(123)BP cluster in the rDNA sequence. NF D is a high-molecular-mass complex containing nucleolin, fibrillarin and U3 and U14 snoRNAs. Furthermore, we show that NF D binds and cleaves pre-rRNA specifically at the P site. Thus we conclude that NF D is a pre-rRNA processing complex that may first assemble on rDNA and then bind nascent pre-rRNA.