Since 1960, nucleoli have been known to play a role in cytoplasmic ribosomal RNA synthesis and ribosome maturation (Birnstiel et al., 1963; Girard et al., 1964; Perry et al., 1960; Vincent et al., 1960).
Isolated nucleoli from rat liver and cancer cells contain RNA polymerase Ⅰ activity (Ro and Busch, 1964; Ro et al., 1964; Ro-Choi, 1995). Purified nucleolar RNAs contain low molecular weight RNAs (4S RNA, 5S RNA, 5.8S RNA, U3 RNA, and 8S RNA) and high molecular weight RNA (28S RNA, 35S RNA, 45S RNA, and > 45S RNA) (Fig. 1). Nucleolar 4S RNAs contain some tRNA(35%) and non-tRNA (65%) (Ritter and Busch, 1971). 5S RNA and 5.8S RNA are ribosomal type RNA. U3 RNA and 8S RNA are nucleolus-specific RNA. U3 RNA has been shown to have a role in 18S rRNA production. 8S RNA has been shown to contain 5.8S RNA sequence associated with it (Reddy et al., 1974). In addition, many minor species of small nucleolar RNA (snoRNA)precipitable with anti-fibrillarin (Fib;34 kDa) antibody have been reported. These Fb(fibrillarin binding) snoRNP RNAs are U3, U8, U13, U14, U15a, U15b, U16a, U16b, U18, U20, U21, Y(U22) and U24-U63 (Foumier and Maxwell, 1993; Smith and Steitz, 1997; Sollner-Webb, 1993). Of these snoRNAs, U3, U8, U13, U22 have 5´ trimethylguanosine cap structures. U14 (intronic RNA of hsc70), U15 (intronic RNA of ribosomal protein S3), U16 and U18 (intronic RNA of ribosomal protein L1), U17a (E1; intronic RNA of RCC1), MRP/7-2, U17b, E2, E3 (elF-4AII mRNA intron), U20 (nucleolin mRNA intron) and U21 (intronic RNA of ribosomal protein L5) are non-capped RNAs. Approximately 200 different snoRNAs are in association with pre-rRNP in the nucleoli (Eichler and Craig, 1994; Smith and Steitz, 1997). Recently, a group of snoRNAs which have 10-21 nucleotides complementary to rRNA and guide 2´-O-methylation of selected nucleotides of rRNA have been reposted in Xenopus and hyman. These include UHG (U22 host gene) encoded U25-U31, U62, and U63 (Tycowski et al., 1996).
Also in human, U48 (63 nt) and U49 (71 nt) have been reported to function for site-specific ribose methylation of pre-ribosomal RNA functioning in site-specific pseudouridine formation has been reported, which include E2, E3, U23, and U64-U72 snoRNAs (Ganot et al., 1977; Smith and Steitz, 1997). Kinetics of RNA labeling with radioactive isotopes and chase experiments demonstrated that ribosomal RNAs (18S RNA, 5.8S RNA, and 28S RNA) are produced from the same precursor 45S RNA synthesized in the nucleolus. The 18RNA is processed rapidly, and transported to the cytoplasm, and not accumulated in the nucleolus. Therefore, stable high nucleolar pre-rRNAs are mostly 28S RNA, 35S RNA, 45S RNA, and >45S RNA. processed 5.8S RNA is hydrogen-bonded to 28S RNa and transported to the cytoplasm together with 5S RNA (Choi, 1985a; 1985b; Epstein et al., 1984; Prestayko et al., 1970) and proteins as ribonucleoprotein (RNP) particles. Ribosomal RNA foldings and assembly of ribosomal protein components (∼45 proteins for the large subunit and ∼33 proteins for the small subunit) are taking place in the nucleolus, nucleus and cytoplasm. RNA polymerase Ⅰ is responsible for 45S RNA and U3 RNA synthesis according to studies done by using thioacetamide (increases nucleolar RNA synthesis) and actinomycin D (inhibits nucleolar RNA synthesis) (Ro-Choe et al., 1976). RNa polymerase responsible for U3 RNA synthesis has also been reported to be Pol Ⅱ or PolⅢ as well (Kiss et al., 1991). Therefore, this is not yet completely resolved. Unlike U1 RNA, which has been reported to have cap trimethylation in the cytoplasm in U snRNA binding protein (sm protein) dependent manner (Mattaj, 1986), U3 RNA trimethylation has been reported to be confined in the nucleus and never goes out to the cytoplasm for maturation (Terns and Dahlberg, 1994). RNA polymerase Ⅲ is responsible for 4S RNA and 5S RNA synthesis.
|