Point Mutations in Yeast CBF5 Can Abolish In Vivo Pseudouridylation of rRNA

Abstract

In budding yeast (Saccharomyces cerevisiae), the majority of box H/ACA small nucleolar RNPs (snoRNPs) have been shown to direct site-specific pseudouridylation of rRNA. Among the known protein components of H/ACA snoRNPs, the essential nucleolar protein Cbf5p is the most likely pseudouridine (⌿) synthase. Cbf5p has considerable sequence similarity to Escherichia coli TruBp, a known ⌿ synthase, and shares the "KP" and "XLD" conserved sequence motifs found in the catalytic domains of three distinct families of known and putative ⌿ synthases. To gain additional evidence on the role of Cbf5p in rRNA biosynthesis, we have used in vitro mutagenesis techniques to introduce various alanine substitutions into the putative ⌿ synthase domain of Cbf5p. Yeast strains expressing these mutated cbf5 genes in a cbf5⌬ null background are viable at 25°C but display pronounced cold-and heat-sensitive growth phenotypes. Most of the mutants contain reduced levels of ⌿ in rRNA at extreme temperatures. Substitution of alanine for an aspartic acid residue in the conserved XLD motif of Cbf5p (mutant cbf5D95A) abolishes in vivo pseudouridylation of rRNA. Some of the mutants are temperature sensitive both for growth and for formation of ⌿ in the rRNA. In most cases, the impaired growth phenotypes are not relieved by transcription of the rRNA from a polymerase II-driven promoter, indicating the absence of polymerase I-related transcriptional defects. There is little or no abnormal accumulation of pre-rRNAs in these mutants, although preferential inhibition of 18S rRNA synthesis is seen in mutant cbf5D95A, which lacks ⌿ in rRNA. A subset of mutations in the ⌿ synthase domain impairs association of the altered Cbf5p proteins with selected box H/ACA snoRNAs, suggesting that the functional catalytic domain is essential for that interaction. Our results provide additional evidence that Cbf5p is the ⌿ synthase component of box H/ACA snoRNPs and suggest that the pseudouridylation of rRNA, although not absolutely required for cell survival, is essential for the formation of fully functional ribosomes.In eukaryotes the biosynthesis of rRNA occurs in a specialized organelle known as the nucleolus (33,41,46,56). rRNA is transcribed by RNA polymerase I (Pol I) as a single large precursor, which undergoes a series of endo-and exonucleolytic cleavages to produce mature rRNA species. In the yeast Saccharomyces cerevisiae, the 35S pre-rRNA precursor is processed to produce mature 18S, 5.8S, and 25S RNAs (54). The 5S rRNA and ribosomal proteins are imported into the nucleolus for assembly into precursors of the 40S and 60S ribosomal subunits before their export to the cytoplasm (16,41,46). An interesting feature of rRNA maturation is the extensive modification the 35S precursor undergoes prior to subsequent cleavage events (29,40,39). One such modification, isomerization of uridine to pseudouridine (⌿), is by far the most abundant posttranscriptional modification of rRNA (29,40,39). Formation of ⌿ is also known to occur in tRNAs (49), small nuclear RNAs (sn...

A CBF5 mutation that disrupts nucleolar localization of early tRNA biosynthesis in yeast also suppresses tRNA gene-mediated transcriptional silencing

“…Subsequently, it was shown to encode a nucleolar antigen (14) with strong homology to NAP57, a rat cell nucleolar protein. Very recently Cbf5p has been shown to be a possible pseudouridine synthetase for rRNA and to be required for efficient processing of rRNA precursors (15,16). Although it was not immediately obvious why a defect in an apparent nucleolar pre-rRNA-processing enzyme might affect tgm silencing, we identified the mutation in CBF5 responsible for this phenotype.…”

Telomerase in the human organism

“…Cbf5p contains the active site for pseudouridine base conversion, initially inferred from the substantial homology of the central region of this protein with other pseudouridine synthases (Koonin, 1996). Interestingly, although null alleles are not viable, substitutions that cripple the Cbf5p active site result in only slow and temperaturesensitive growth (Zebarjadian et al, 1999). This suggests that the binding of H/ACA RNPs to RNA targets may be more important than the subsequent base modi®cation.…”

Mitochondrial Myopathy and Sideroblastic Anemia (MLASA)

“…This is not the first human disorder that can be attributed to a defect in pseudouridylation. One form of dyskeratosis congenita (X-linked) has been shown to be due to the loss or mutation of the dyskerin protein (40), which is a homologue of the yeast protein Cbf5p, the pseudouridine synthase responsi- ble for ribosomal RNA modification (41). Dyskerin is part of the telomerase complex, and its loss results in a destabilization of the complex (42).…”