Transcription of tRNA

Fluorescent labeling of D residues in native and transcripts of tRNAs. 

Three general transcription factors are required for Pol III to Initiate transcription of tRNA and SS-rRNA genes In vitro. Two multimeric factors, TFIIIC and TFIIIB, participate In Initiation at both tRNA and SS-rRNA promoters a third factor, TFIIIA, Is required for initiation at SS-rRNA promoters. As with assembly of Pol I and Pol II Initiation complexes, the Pol III general transcription factors bind to promoter DNA in a defined sequence. 

S Ribosomal RNA genes - The most thoroughly studied genes transcribed by RNA polymerase III are those for 5S ribosomal RNA. At least three protein factors in addition to polymerase III are needed for the expression of the 5S rRNA genes in vitro. Two of these transcription factors (TFIIIB and TFIIIC) appear to participate in the transcription of tRNA genes as well, but one (called TFIIIA) is specific for the 5S genes. 

As described in the preceding sections protein synthesis involves transcription of the DNA to rtiRNA followed by translation of the mRNA as an amino acid sequence In addition to outlining the mechanics of transcription we have described the relationship among mRNA codons tRNA anticodons and ammo acids... 

There are three distinct nuclear DNA-dependent RNA polymerases in mammals RNA polymerases I, II, and III. These enzymes control the transcriptional function—the transcription of rRNA, mRNA, and small RNA (tRNA/5S rRNA, snRNA) genes, respectively. 

TFIIIC subunits TFIIIC90 and TFIIICllO. The known function of TFIIIC is to initiate transcription complex formation by binding to promoter DNA and recruiting TBP-containing TFIIIB and RNA polymerase III, that directs synthesis of tRNA precursors. TFIIIC have an intrinsic HAT activity. 

A new method to efficiently express prokaryotic tRNAs in yeast involves an external Pol III promoter containing the consensus A- and B-box sequences (Figure 5(c)). When placed upstream of the E. coli tRNA (without the 3 -CCA trinucleotide), the promoter drives transcription of a primary RNA transcript consisting... 

The mRNA is bound to the smaller 30S subunit of the bacterial ribosome. The mRNA is a transcription of one of the genes of DNA, and carries the information as a series of three-base codons. The message is read (translated) in the 5 to 3 direction along the mRNA molecule. The aminoacyl-tRNA anticodon (UAC) allows binding via hydrogen bonding to the appropriate codon (AUG) on mRNA. In prokaryotes, the first amino acid encoded in the sequence is A-formylmethionine (fMet). Although the codon for initiation (A-formylmethionine) is the same as... 

Initiation of reverse transcription in HIV-infected cells relies on a critical RNA-RNA interaction between tRNA y s, which is preferentially packaged into the viral particle, and a specific viral RNA seqnence. The 3 -terminaI 18 nucleotides of tRNA y are complementary to the primer binding site (PBS) sequence located in the 5 -Iong terminal repeat (LTR) of the viral RNA genome (Figure 10.3). The UUU anticodon of the tRNA is complementary to and binds to an adenosine rich loop located 8 nucleotides upstream (5 ) of the PBS. This RNA-RNA duplex which is formed when tRNA y s binds to the PBS fits within the active site of HIV-1 reverse transcriptase, bnt mnitiple interactions between the viral RNA and tRNA y are necessary for efficient initiation of reverse transcription. This interaction nucleates the reverse transcription complex which contains viral RNA, reverse transcriptase, tRNA y pl , nncleocapsid p7, and Vpr (Viral protein R), as well as multiple host factors." ... 

The primary transcripts of prokaryotic and eukaryotic tRNAs are processed by the removal of sequences from each end (cleavage) and in a few cases by the removal of introns (splicing). Many bases and sugars in tRNAs are also modified mature tRNAs are replete with unusual bases not found in other nucleic acids (see Fig. 26-24). 

A primary transcript is a linear copy of a transcriptional unit—the segment of DNA between specific initiation and termination sequences. The primary transcripts of both prokaryotic and eukaryotic tRNAs and rRNAs are post-transcriptionally modified by cleavage of the original transcripts by ribonucleases. tRNAs are then further modified to help give each species its unique identity. In contrast, prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic nrRNA is extensively modified posttranscriptionally. 

The structure of the precursor to the minor E. coli tyrosine tRNA, is shown in Fig. 28-10. This is encoded by the amber suppressor gene SupF (see Chapter 29). Transcription of its gene is initiated by GTP 43 bp upstream of the 5 end of the mature tRNA and usually terminates at a p-dependent signal 225 bp beyond the CCA terminus of the tRNA. An endonuclease cuts the transcript a few nucleotides beyond the CCA end. It is then trimmed to an 130-nucleotide piece still containing 2-3 extra nucleotides at the 3 end. This intermediate is cut by RNase P at the 5 end after which final trimming is done at the 3 end (Fig. 28-10).226 227... 

Not all aminoacyl-tRNA synthetases have editing sites. The cysteinyl- and tyrosyl-tRNA synthetases bind the correct substrates so much more tightly than their competitors that they do not need to edit.13,14 Similarly, since the accuracy of transcription of DNA by RNA polymerase is better than the overall observed error rate in protein synthesis at about 1 part in 104, RNA polymerases do not need to edit.15 The same should be true for codon-anticodon interactions on the ribosome. However, it is possible that accuracy has been sacrificed to achieve higher rates in this case, which is analogous to a change from Michaelis-Menten to Briggs-Haldane kinetics, and so an editing step is required.16... 

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