|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.
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.
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.