Yong C. Choi

Low-Molecular-Weight Nuclear RNAs

[1, p. 285; 2-14]. Some RNAs smaller in size than these which contain dihydrouridine or dihydrothymidine have also been reported to be present in the nucleus, specifically in association with the chromatin fraction [15-20]. Although the functions of most of these LMWN RNAs are not defined, their specificity of localization in the nucleolus and the nuclear chromatin fraction along with the number of these molecules (which approximates that of the number of functional genes) has suggested they may have an initiating or structural role in gene readouts or a regulatory role in gene function [8, 10, 2123]. The recent findings which have indicated that some low-molecular-weight nucleolar RNAs have specific temporal associations with newly synthesized ribosomal precursors suggest the possibility that they may be involved in specific phases of synthesis or modification of the ribosomal precursors [24]. Number of low-molecular-weight nuclear RNAs.—In a search for a definite role for these molecules in cell function, one of the critical problems has been the clarification of the number of types of these RNA molecules. Although it is not yet possible to specify with precision the total number of low-molecular-weight RNA species in the various fractions of the nucleus, the minimal estimate is eleven (fig. 1), and the maximum is approximately forty-three [1, 25]. None of these values, however, exclude possible microheterogeneity of the various nuclear RNA species. Table 1 indicates that there are at least

Studies on the role of 23 s nucleolar RNA as an intermediate in the synthesis of 18 s ribosomal RNA

Abstract From whole nucleolar RNA of Novikoff hepatoma cells, a single symmetrical peak of 23 s RNA was obtained by repeated sucrose density-gradient centrifugations. Hybridization experiments showed that nucleolar 23 s RNA is composed of 18 s ribosomal RNA and other RNA segments. Further, nucleolar 45 s RNA is composed of nucleolar 23 s RNA and other RNA segments. The purified nucleolar 23 s RNA contained almost the same percentages and numbers of each of the alkali stable dinucleotides as 18 s rRNA; these differ markedly from the corresponding values for the alkali stable dinucleotides of 28 s rRNA. The 5′-terminal nucleotide of both nucleolar 23 s RNA and 18 s rRNA is pGp. However, the major 3′-terminal nucleosides of 18 s rRNA and 23 s nucleolar RNA are adenosine and uridine, respectively. From these results, it is concluded that nucleolar 23 s RNA contains ribosomal 18 s RNA in its 5′-terminal portion and is a cleavage product of nucleolar 45 s RNA.

Genetic Engineering and Cancer Chemotherapy

Studies on the composition of newly-synthesized nucleolar and ribosomal RNA of the Novikoff rat hepatoma have demonstrated that a portion of these molecules differs from the corresponding portion of s

Comparison of large fragments obtained by T1 RNase digestion of ribosomal and nucleolar preribosomal RNA of novikoff hepatoma ascites cells: The 5′-terminal eicosanucleotide

Summary Large T 1 RNase fragments of 18S and 28S rRNA and their nucleolar precursors were separated by successive elution from columns of DEAE Sephadex at pH 7.6 and DEAE-cellulose at pH 3.4, respectively. The fragments that contain the methylated oligonucleotides Am-Gm-Cm-Ap, Um-Gm-Up, Gm-Up and Cm-Ap are referred to as α2 −2 , α2 −4 , α2 −3 and α1 −1 ′. Fragment α2 −2 is the eicosanucleotide Am-Gm-Cm-A-A-A-U-U-C-A-U-A-U-U-C-A-A-A-C-Gp which is the 5′ end of 28S rRNA and its precursors including nucleolar 45S RNA. The dinucleotide Cm-Ap is present only in 18S rRNA and 45S nucleolar RNA.

Effect of Sarkosyl on the fidelity of preribosomal RNA synthesis in isolated nucleoli

Abstract When nucleoli isolated from Novikoff hepatoma ascites cells were treated with 2% Sarkosyl, 95% of the nucleolar proteins were extracted. In addition to RNA polymerase I, a heterogeneous group of proteins remained bound to DNA. The homochromatography fingerprinting analysis of the transcripts of the Sarkosyl pellet had virtually all of the marker oligonucleotides that are found in transcripts from whole nucleoli. In addition, seven discrete spots were present which were not found either in transcripts from intact nucleoli or in 45S pre-rRNA labeled in vivo . These results suggest that the endogenous RNA polymerase I present in the Sarkosyl pellet may have transcribed sequences juxtaposed to pre-rRNA coding region in addition to faithfully transcribing the ribosomal genes. It is possible that Sarkosyl extracted factor(s) necessary for proper termination or processing of pre-rRNA.

Ultrastructural and Purification Studies on Human Tumor Nucleolar Antigens and Nucleolar Particles

The presence of common nucleolar antigens in a broad array of human malignant tumors has led to several lines of investigations. In addition to studies on an increasing number of benign and malignant neoplasms with a variety of antibodies designed to statistically evaluate the presence of nucleolar antigens, purification procedures and chemical analyses are being used to characterize the specific antigens. The localization of the nucleolar antigens in HeLa cells was studied by the postembedding immunoelectron microscopic procedure employing rabbit antibodies to nucleoli or nuclear Tris extracts of these cells. The products of the peroxidase-antiperoxidase complexes visualized by the reaction with diaminobenzidine in nucleoli were mainly found in the nucleolonemas which contain the dense nucleolar RNP components. When these nucleoli became compact after treatment of HeLa cells with adriamycin, the distribution of the immunoreactive products was altered along with distribution of the dense nucleolar components. The human nucleolar antigens were mainly localized to nucleolar regions containing the nucleolar RNP components. Improved purification of the antigens made it possible to provide a satisfactory amino acid analysis of one pI 6.3 antigen. Interestingly, some of the nucleolar antigen was found in miniparticle undescribed until now.

Methylation of snRNAs and 18S rRNA of Novikoff heptoma

In the biosynthesis of mature snRNAs and rRNAs, RNA methylation is one of the important mechanisms involved in their posttranscriptional modifications. The processes of RNA methylation are catalyzed by RNA methylases which specifically recognize methylation sites. This enzymatic recognition of methylating sites is specific for particular species of RNA and dictates the types, numbers of methylated nucleotide sequences and their order in maturation stages (Perry, 1976).

HIERARCHICAL CONTROLS OF NUCLEOLAR SYNTHETIC FUNCTIONS

Abstract: Nucleoli of cancer cells have high rates of rRNA synthesis. A variety of controls affect the fidelity and rates of rRNA synthesis as well as the availability of rDNA. Studies on the roles of nucleolar proteins show that proteins maintaining fidelity of nucleolar transcription are soluble in 0.6 M NaC1. Of the proteins decreased with increased nucleolar activity, nucleosomal protein A24 is an interesting conjugate of ubiquitin and histone 2A susceptible to specific cleavage by a specific proteinase (Eickbush et al , Cell 9 , 785, 1977). Moment-to-moment control of rates of nucleolar rDNA readouts may be related to cytoplasmic factors such as elongation factor EF-1 which we recently found in nuclei and nucleoli.