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A3 Ribosomal RNA (rRNA)

A3 Ribosomal RNA (rRNA). Ribosomes( 核糖体 ) Transcription and processing of rRNA genes in prokaryotes Transcription and processing in eukaryotes. 1. Ribosomes. Macromolecules composed of rRNA bound to protein. Function : bind to mRNA and transcribe it to produce protein

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A3 Ribosomal RNA (rRNA)

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  1. A3 Ribosomal RNA (rRNA)

  2. Ribosomes(核糖体) • Transcription and processing of rRNA genes in prokaryotes • Transcription and processing in eukaryotes

  3. 1. Ribosomes • Macromolecules composed of rRNA bound to protein. • Function: bind to mRNA and transcribe it to produce protein • Quantity: large numbers. A typical bacterium contains 20, 000 ribosomes which account for 80% of its RNA and 10% of its protein

  4. S-value: Svedberg units. Determined by the size, shape and macro-molecular structure of the ribosome • Structure: • In prokaryotes, 70S (50S large subunit- --contains 23S and 5S rRNAs and 31 polypeptides+30S---small subunit contains 16S rRNA and 21 polypeptides).

  5. In eukaryotes, 80S (60S subunit---contains 28S, 5.8S and 5S rRNAs and 49 polypeptides , 40S---contains 18S rRNA and about 33 proteins) • In ribosomes, RNA adopt a characteristic 3-D structure to provide a frame work for proteins to attach. Some of the RNA molecules also are ribozymes.

  6. 2.Transcription and processing of rRNA genes in prokaryotes • rRNA genes present in the genome as multiple copies (in E. coli, seven), and • transcribes as a single pre-rRNA molecule: 16S-23S-5S, and between one and four tRNA sequences are present in each gene.

  7. rRNA processing : folds to form stem-loop structures→ ribosomal proteins bind to the folded rRNA, modified by methylation→ RNase Ⅲ cleavage→ cleavage by M5, M16 and M23

  8. 3. Transcription and processing of rRNA genes in eukaryotes • rRNA genes: 28S,18S and 5.8S rDNA sequences are present in a single gene which exists as multiple copies (human, 200=5 cluster ×40 copies) separated from each other by short nontranscribed regions (Fig.)

  9. Transcription occurs in nucleolus by RNA polymeraseⅠ

  10. Processing :similar to prokaryotes. Methylation is catalyzed by molecules composed of snRNPs (small nuclear ribonucleoprotein). In human, pre-rRNA is 45S. Initial cleavage at external transcribed spacers (ETSs) , following at internal transcribed spacers (ITSs) to produce 20S and 32S precursors

  11. further cleavage produces 28S, 18S and 5.8S rRNAs, 5.8S base pairs with 28S.

  12. 5S rRNA is transcribed separately by the RNA polymerase Ⅲ from unlinked gene as a short 121base transcript which does not undergo processing.

  13. Key points • The structure of ribosomes • How rRNA transcript is processed in eukaryote?

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