NSR1 is required for pre-rRNA processing and for the proper maintenance of steady-state levels of ribosomal subunits.

Abstract

NSR1 is a yeast nuclear localization sequence-binding protein showing striking similarity in its domain structure to nucleolin. Cells lacking NSR1 are viable but have a severe growth defect. We show here that NSR1, like nucleolin, is involved in ribosome biogenesis. The nsrl mutant is deficient in pre-rRNA processing such that the initial 35S pre-rRNA processing is blocked and 20S pre-rRNA is nearly absent. The reduced amount of 20S pre-rRNA leads to a shortage of 18S rRNA and is reflected in a change in the distribution of 60S and 40S ribosomal subunits; there is no free pool of 40S subunits, and the free pool of 60S subunits is greatly increased in size. The lack of free 40S subunits or the improper assembly of these subunits causes the nsrl mutant to show sensitivity to the antibiotic paromomycin, which affects protein translation, at concentrations that do not affect the growth of the wild-type strain. Our data support the idea that NSR1 is involved in the proper assembly of pre-rRNA particles, possibly by bringing rRNA and ribosomal proteins together by virtue of its nuclear localization sequence-binding domain and multiple RNA recognition motifs. Alternatively, NSR1 may also act to regulate the nuclear entry of ribosomal proteins required for proper assembly of pre-rRNA particles.In eukaryotic cells, the nucleolus is a specialized subcompartment in which ribosome biogenesis occurs. Pre-rRNA first is transcribed from rDNA genes located in the nucleolus and then undergoes a series of modifications. Ribosomal proteins are imported from the cytoplasm and packed onto the RNA molecule. At the same time, the primary transcript goes through sequential cleavages to generate mature forms of rRNA. Finally, the newly formed ribosomal particles are exported to the cytoplasm.In the yeast Saccharomyces cerevisiae, the largest detectable transcript from the rDNA genes is a 35S pre-rRNA (17), which is rapidly processed into three molecules: the 18S rRNA assembled into 40S ribosomal subunits and the 5.8S and 25S rRNAs found in 60S subunits (Fig. 1). During or immediately after transcription of the rDNA genes, the pre-rRNA is modified, mainly by methylation (32, 34). Meanwhile, a large number of ribosomal proteins and nonribosomal components (including proteins and RNA) associate with the pre-rRNA to form a 90S preribosomal particle. This particle is then split into 66S and 43S preribosomal particles, which are the precursors of the 60S and 40S subunits, respectively (31). While the 66S particles mature completely within the nucleus, the final maturation steps of the 43S particles, including the processing of 20S to 18S rRNA, are completed in the cytoplasm (31,33).Only a few mutations that block specific steps in the pre-rRNA processing pathway have been found. One example is a temperature-sensitive mutation in a gene designated RRPJ that was shown to prevent the 27S-to-25S rRNA cleavage step specifically (1). In another mutant, CLP-8, the efficiency of the processing of 20S to 18S rRNA is greatly reduced, apparently because...

An abundant nucleolar phosphoprotein is associated with ribosomal DNA in Tetrahymena macronuclei.

“…These data suggest that Nopp52, nucleolin, and NSR1 are members of a protein family with potentially related functions. However, nucleolin does not rescue the slow growth phenotype of yeast NSR1 mutants (Lee et al, 1992) and, therefore, complete functional identity cannot be assumed from structural similarities.…”

“…Both nucleolin and NSR1 appear to be involved in prerRNA processing. In yeast NSR1 mutants, there is a disruption of pre-rRNA processing (Kondo and Inouye, 1992;Lee et al, 1992). Nucleolin is associated with pre-rRNA (Herrera and Olson, 1986) and this association appears to involve both the RRMs and the GRF-rich regions (Sapp et al, 1989;Ghisolfi et al, 1992).…”

Characterization of AtNUC-L1 Reveals a Central Role of Nucleolin in Nucleolus Organization and Silencing of AtNUC-L2 Gene in Arabidopsis

“…Mutations of genes encoding nucleolin homologue in budding (NSR1) and fission (GAR2) yeast produce severe effects on cell growth and disrupt balanced production of the small and large ribosomal subunits (Lee et al, 1992;Gulli et al, 1995). Moreover, it has been shown that the nucleolar structure in gar2 cells is dramatically reorganized and unusual structures are detected (Leger-Silvestre et al, 1997).…”

DRS1 to DRS7, novel genes required for ribosome assembly and function in Saccharomyces cerevisiae.

“…Striking changes in the levels of 60S subunits have been observed in strains having a mutation in either the SPB4 or RRP1 gene, whose products are involved in processing of the 27S precursor to the mature 25S rRNA (1,45). Strains bearing a null allele of NSR1 were shown to have defects in processing of the 20S rRNA precursor to 18S rRNA as well as changes in the steady-state level or distribution of 40S ribosomal subunits (28). Mutations in two genes encoding translation factors, GCD2 (14), and SUII (67), affect steady-state levels of 80S monoribosomes and polyribosomes.…”