In the study of cellular RNA chemistry, a major thrust of research focused upon sequence determinations for decades. Structures of snRNAs (4.5S RNA I (Alu), U1, U2, U3, U4, U5, and U6) were determined at Baylor College of Medicine, Houston, Tex, in an earlier time of pregenomic era. They show novel modifications including base methylation, sugar methylation, 5 -cap structures (types 0–III) and sequence heterogeneity. This work offered an exciting problem of posttranscriptional modification and underwent numerous significant advances through technological revolutions during pregenomic, genomic, and postgenomic eras. Presently, snRNA research is making progresses involved in enzymology of snRNA modifications, molecular evolution, mechanism of spliceosome assembly, chemical mechanism of intron removal, high-order structure of snRNA in spliceosome, and pathology of splicing. These works are destined to reach final pathway of work “Function and Structure of Spliceosome” in addition to exciting new exploitation of other noncoding RNAs in all aspects of regulatory functions. 1. Introduction A key element in the study of cellular RNA metabolism is the molecular characterization of RNA. This characterization requires accurate determination of the RNA sequence. It is imperative to understand how RNA structure complements the functional definition of RNA. Cellular RNAs are posttranscriptionally modified at various points in the primary RNA transcript as well as processed. In cellular RNA metabolisms, RNA maturation is performed through various structural alterations that include chemical modifications of constituent components. A most representative modification is observed in chain shortening, rearrangements by transfer of phosphodiester linkages involved in splicing mechanisms (pre-mRNA), deletions (pre-rRNA), and transsplicing (trypanosomal mRNA). Another is chain expansion demonstrated by modifications observed on polyadenylation, U-addition at 3′ ends, 5′-cap formation at 5′ ends, and insertions within trypanosome RNA. Other examples of modifications are base modifications, such as deaminations, methylations, hypermodifications, and ribose methylations. The most modified RNAs are tRNAs containing approximately 2–22 modified nucleotides per molecule of ~75 nucleotide length, and there have been more than 130 different signature modified nucleotides reported [1]. The discovery of snRNA and m 3 2 . 2 . 7 G caps occurred within the last 50 years. They also contain their own specific modified nucleotides such as Ψ, m6A, m2G, and 2′-O-methylated nucleotides
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