RNA Splicing in Metazoans

Content of Nucleic Acid Synthesis
» Nucleic Acids
» Structure and Function of Nucleic Acids
    » Basic Chemical Structure
    » Base Pairing in Nucleic Acids: Double Helical Structure of Dna
    » Size, Structure, Organization, and Complexity of Genomes
    » Information Storage, Processing, and Transfer
    » Chromosomal Dna Compaction and Its Implications in Replication and Transcription
    » DNA Sequence and Chromosome Organization
    » Repetitive Sequences: Selfish DNA
    » Chromatin Remodeling and Histone Acetylation
» Nucleic Acid Syntheses
    » Similarity of DNA and RNA Synthesis
    » DNA Replication Vs Transcription: Enzymatic Processes
    » Multiplicity of DNA and RNA Polymerases
» DNA Replication and Its Regulation
    » DNA Replication
    » Regulation of DNA Replication
    » Regulation of Bacterial DNA Replication at the Level of Initiation
    » DNA Chain Elongation and Termination in Prokaryotes
    » General Features of Eukaryotic DNA Replication
    » Licensing of Eukaryotic Genome Replication
    » Fidelity of DNA Replication
    » Replication of Telomeres—The End Game
    » Telomere Shortening: Linkage Between Telomere Length and Limited Life Span
» Maintenance of Genome Integrity
» DNA Manipulations and their Applications
» Transcriptional Processes
    » Recognition of Prokaryotic Promoters and Role of S-Factors
    » Regulation of Transcription in Bacteria
    » Eukaryotic Transcription
    » RNA Splicing in Metazoans
    » Regulation of Transcription in Eukaryotes
    » Fidelity of Transcription (RNA Editing)
» Chemical Synthesis of Nucleic Acids (Oligonucleotides)
» Bibliography of Nucleic Acid Synthesis
The central dogma of molecular biology that the information flowfromDNAtoRNAto protein involves colinearity of the sequences of the monomer units is somewhat violated in metazoans because of the presence of interrupted or fragmented genes (Fig. 8). Thus, while the polypeptide sequence is colinear with the codons of the coding sequence in the mRNA, the RNA itself is not collinear with the gene from which it is transcribed. In other words, the gene contains additional intervening sequences called introns, which are transcribed but whose RNA sequence is subsequently removed from the final mRNA containing the coding sequence. The primary gene transcripts of nuclear genomes, called heterogeneous nuclear RNA (hnRNAs), are present in a form of protein-bound particles (ribonucleoprotein particles, or hnRNP). RNA splicing is the process of excising introns from hnRNAs, and contiguous exons are then joined to form mature mRNAs, which are subsequently translocated to cytoplasm and are used as templates for translation (Fig. 8). The cleavage and rejoining occur at specific junctions between exons and introns, so that there are no errors in mature mRNA. First, two adjacent exons are aligned, while the intervening intron is extruded, forming a loop (“lariat”) structure. Then the upstream exon is cleaved and joined to the downstream exon via a
FIGURE 8 A schematic representation of
RNA splicing. The coding sequence in
metazoan genomes is usually present in
segments (exons; indicated by boxes)
interspersed between noncoding introns.
After synthesis of the primary RNA
transcript (called heterogeneous nuclear
RNA or hnRNA), the intron sequences
are removed by precise cleavage and
rejoining is mediated by the spliceosome
complex, so that the resulting mature mRNA
contains a correctly juxtaposed coding
sequence for the polypeptide. The mRNA is
also “capped” by 5´-5´ linkage with GMP,
and a tail of poly(A) is added at the 3´
terminus to increase the stability of
mRNA and to enhance its efficiency in
directing protein synthesis when the mRNA
is transported from the nucleus to the
transesterification reaction. In most cases, two factors are essential for this process. One, the cis-elements in introns and exons, is the signaling sequences for the exact junction sites. The other is the splicing machinery, consisting of several small ribonucleoprotein particles (snRNP; U1, U2, and U4–U6), each of which contains small RNA molecules and proteins. The U1 and U2 snRNPs contain RNA complementary to the intron ciselement and catalyze the formation of the intron lariat, while two adjacent exons are aligned together.With other snRNPs forming an intermediate complex (spliceosome), U6 catalyzes the transesterfication. It should be noted that introns in RNA of some lower eukaryotic species are autospliced and therefore do not require snRNPs.

Termination of eukaryotic transcription is coupled with processing. The mature rRNA is obtained by cleavage of a larger primary transcript synthesized by Pol I. Termination of Pol II transcription occurs at a repeat sequence of U, as in the case of E. coli RNA polymerase, but without the presence of a hairpin structure. More importantly, the 3´ termini of mRNAs are generated by cleavage of primary precursor transcripts followed by addition of a tail of poly(A), a homopolymer of up to several hundred AMP residues synthesized by poly(A) polymerase in a template-independent reaction.