Rcl1p, the yeast protein similar to the RNA 3 '-phosphate cyclase, associates with U3 snoRNP and is required for 18S rRNA biogenesis

RNA 3'-terminal phosphate cyclases are evolutionarily conserved enzymes cat alysing conversion of the 3'-terminal phosphate in RNA to the 2',3'-cyclic phosphodiester, Their biological role remains unknown. The yeast Saccharomy ces cerevisiae contains a gene encoding a protein with strong sequence simi larity to the characterized cyclases from humans and Escherichia coli, The gene, named RCL1 (for (R) under bar NA terminal phosphate (c) under bar ycl ase (l) under bar ike), is essential for growth, and its product, Rcl1p, is localized in the nucleolus. Depletion or inactivation of Rcl1p impairs pre -rRNA processing at sites Ao, Al and Az, and leads to a strong decrease in 18S rRNA and 40S ribosomal subunit levels. Immunoprecipitations indicate th at Rcl1p is specifically associated with the U3 snoRNP, although, based on gradient analyses, it is not its structural component. Most of Rcl1p sedime nts in association with the 70-80S pre-ribosomal particle and a 10S complex of unknown identity. Proteins similar to Rcl1p are encoded in genomes of a ll eukaryotes investigated and the mouse orthologue complements yeast strai ns depleted of Rcl1p, Possible functions of Rcl1p in pre-rRNA processing an d its relationship to the RNA 3'-phosphate cyclase are discussed.