Characterization of a crucifer plant pre-rRNA processing complex.

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.