MoBio Transcription of RNA Genes Chapter 4

The products of RNA genes include rRNA, tRNA and small RNA molecules. In the mammalian genome, the three rRNA genes, 28S, 18S, and 5.8S, are clustered as a pre-rRNA gene. which is transcribed by RNA polymerase I. tRNA, 5S rRNA and the U6 snRNA genes are transcribed by RNA polymerase III. Most snRNA genes are transcribed by RNA polymerase II using the same mechanism as transcribing protein genes.

Transcription by RNA Polymerase I

RNA polymerase I is devoted to the synthesis of pre-rRNA. Like RNA polymerase II, a pre-initiation complex (PIC) has to be formed before RNA polymerase I can exert its function. The order of the PIC assembly is illustrated in Figure 4-I-1.

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Figure 4-I-1. The PIC assembly for RNA polymerase I. The regulatory region of pre-rRNA gene contains a core element and an upstream control element (UCE). Binding of two upstream binding factors (UBF) to both elements may induce DNA looping, and subsequently recruiting TATA-binding protein (TBP) and TBP-associated factors (TAFI). Finally, RNA polymerase I joins the complex and completes the assembly process.

Transcription by RNA Polymerase III

Transcription of tRNA gene

The regulatory region of tRNA gene contains A box and B box located inside the transcription unit. The PIC assembly begins with the binding of TFIIIC to both elements.

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Figure 4-I-2. The PIC assembly for the transcription of tRNA gene.

Transcription of 5S-rRNA gene

The regulatory region of 5S-rRNA gene contains a C box, also located inside the transcription unit. The PIC assembly begins with the binding of TFIIIA to the C box.

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Figure 4-I-3. The PIC assembly for the transcription of 5S-rRNA gene. Note that TBP is involved in the transcription by all three types of RNA polymerases in eukaryotes.