Processing of 5S RNA and tRNAs

RNA polymerase I is located in the nucleolus and synthesizes a large precursor that is later processed to form rRNA. It is completely resistant to inhibition by a-amanitin. RNA polymerase II is located in the nucleoplasm and synthesizes large precursor RNAs (sometimes called heterogeneous nuclear RNA, or hnRNA) that are processed to form cytoplasmic mRNAs. It is also responsible for the synthesis of most viral RNA in virus-infected cells. PolII is very sensitive to a-amanitin, being inhibited by 50% at 0.05 /u,g/ml. RNA polymerase III is also located in the nucleoplasm and synthesizes small RNAs, such as 5S RNA and the precursors to tRNAs. This enzyme is somewhat resistant to a-amanitin, requiring about 5 /u,g/ml to reach 50% inhibition. 

Transcription is catalyzed by DNA-dependent RNA polymerases. These act in a similar way to DNA polymerases (see p. 240), except that they incorporate ribonucleotides instead of deoxyribonucleotides into the newly synthesized strand also, they do not require a primer. Eukaryotic cells contain at least three different types of RNA polymerase. RNA polymerase I synthesizes an RNA with a sedimentation coef cient (see p. 200) of 45 S, which serves as precursor for three ribosomal RNAs. The products of RNA polymerase II are hnRNAs, from which mRNAs later develop, as well as precursors for snRNAs. Finally, RNA polymerase III transcribes genes that code for tRNAs, 5S rRNA, and certain snRNAs. These precursors give rise to functional RNA molecules by a process called RNA maturation (see p. 246). Polymerases II and III are inhibited by a-amanitin, a toxin in the Amanita phalloides mushroom. 

Posttranscriptional processing is not limited to mRNA. Ribosomal RNAs of both prokaryotic and eukaryotic cells are made from longer precursors called preribosomal RNAs, or pre-rRNAs, synthesized by Pol I. In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing (Fig. 26-21). 

RNA polymerase Iff is found in the nucleoplasm and synthesizes the tRNAs, precursors of 5S ribosomal RNA, and a variety of other small RNA molecules involved in mRNA processing and protein transport. 

No. Eukaryotic RNA polymerases have been isolated from many tissues, and in all cases three distinct enzymes have been found in the nucleus. All are complex in structure and contain a number of polypeptide subunits. RNA polymerase I is known to be involved specifically in the transcription of rRNA genes. RNA polymerase II gives rise to transcripts that are subsequently processed to yield mRNA. RNA polymerase III is responsible for the transcription of the tRNA genes and a small ribosomal RNA gene that yields a species called 5S RNA. There is also a distinct RNA polymerase found in mitochondria, which is similar to bacterial RNA polymerase. 

In eukaryotes, three RNA polymerases have been identified. RNA polymerase I, located in the nucleolus (Section 9.4), synthesizes 5.8S, 18S, and 28S rRNA. RNA polymerase II, found in the nucleoplasm, participates in mRNA synthesis. RNA polymerase III, also identified within the nucleoplasm, functions in tRNA and 5S rRNA synthesis. Post-transcriptional processing similar to that of prokaryotic transcripts produces rRNAs and tRNAs. 

All three species of E. coli rRNAs are derived from a single transcript which contains the rRNAs in the order of 16S—23S—5S. The transcript also contains one or more intermittent tRNAs. In E. coli, there are seven such rRNA transcription units dispersed throughout the genome and arranged in rrn operons. The processing of the spacer RNAs between each rRNA and tRNA is carried out by a series of RNases. 

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