Nop2p Is Required for Pre-rRNA Processing and 60S Ribosome Subunit Synthesis in Yeast

Abstract

To investigate the function of the nucleolar protein Nop2p in Saccharomyces cerevisiae, we constructed a strain in which NOP2 is under the control of a repressible promoter. Repression of NOP2 expression lengthens the doubling time of this strain about fivefold and reduces steady-state levels of 60S ribosomal subunits, 80S ribosomes, and polysomes. Levels of 40S subunits increase as the free pool of 60S subunits is reduced. Nop2p depletion impairs processing of the 35S pre-rRNA and inhibits processing of 27S pre-rRNA, which results in lower steady-state levels of 25S rRNA and 5.8S rRNA. Processing of 20S pre-rRNA to 18S rRNA is not significantly affected. Processing at sites A 2 , A 3 , B 1L , and B 1S and the generation of 5 termini of different pre-rRNA intermediates appear to be normal after Nop2p depletion. Sequence comparisons suggest that Nop2p may function as a methyltransferase. 2-O-ribose methylation of the conserved site UmGm⌿UC 2922 is known to take place during processing of 27S pre-rRNA. Although Nop2p depletion lengthens the half-life of 27S pre-RNA, methylation of UmGm⌿UC 2922 in 27S pre-rRNA is low during Nop2p depletion. However, methylation of UmGm⌿UC 2922 in mature 25S rRNA appears normal. These findings provide evidence for a close interconnection between methylation at this conserved site and the processing step that yields the 25S rRNA.

Identification of a role for Saccharomyces cerevisiae Cgr1p in pre-rRNA processing and 60S ribosome subunit synthesis

“…Each of the two cgr1 mutants generated for this study was deficient in the steady-state levels of 60S ribosome subunits, resulting in abnormally high levels of the 40S subunit and an increase in half-mer polysomes. Similar profiles have been described for mutants defective in components required for pre-rRNA processing or 60S subunit assembly (Ripmaster et al, 1992 ;Hong et al, 1997 ;de la Cruz et al, 1998a ;Basu et al, 2001), suggesting that Cgr1p is also involved in this pathway.…”

“…and A $ (Russell & Tollervey, 1992). This pathway is infrequently used in wild-type yeast, but increased levels of the 23S rRNA have been associated with depletion of other proteins involved in pre-rRNA processing (Tollervey et al, 1991 ;Girard et al, 1992 ;Hong et al, 1997). Taken together, these results indicate that the primary defect associated with loss of Cgr1p function, either through protein truncation or depletion, is in the formation of the 25S rRNA from the 27S pre-rRNA intermediate.…”

“…Subsequent 3h 5h exonucleolytic digestion of the 7S rRNA generates the mature 5n8S rRNA. Figure modified with permission from Hong et al (1997).…”

Myb-binding Protein 1a (Mybbp1a) Regulates Levels and Processing of Pre-ribosomal RNA

“…4D). We identified a large variety of ribosomal proteins of both the small and large ribosome subunits as well as of several other factors such as fibrillarin, DDX21, Rrs1, Ebp2, and Nol1 (yeast Nop2) to which roles in rRNA processing and ribosome assembly have been assigned in previous studies (56,(63)(64)(65)(66). This result was supported by the observation that Mybbp1a co-migrated with the small and large ribosome subunit pre-ribosomal particles on a polysome sucrose gradient (Fig.…”

The 58‐kDa microspherule protein (MSP58), a nucleolar protein, interacts with nucleolar protein p120

“…A recent study showed that NOP2 is During telophase, prenucleolar bodies and other components aggregate to form nucleoli [5]. It is unclear which nucleolar ele-required for pre-rRNA processing and 60 S ribosomal synthesis in yeast [12]. ments are responsible for triggering the assembly of prenucleolar bodies and nucleolar reformation [6].…”