RNA polymerase III

In eukaryote cells, RNA polymerase III (also called Pol III) transcribes DNA to synthesize ribosomal 5S rRNA, tRNA and other small RNAs.

The genes transcribed by RNA Pol III fall in the category of "housekeeping" genes whose expression is required in all cell types and most environmental conditions. Therefore, the regulation of Pol III transcription is primarily tied to the regulation of cell growth and the cell cycle, thus requiring fewer regulatory proteins than RNA polymerase II. Under stress conditions however, the protein Maf1 represses Pol III activity.

In the process of transcription (by any polymerase) there are three main stages:

Initiation: the construction of the polymerase complex on the promoter. Pol III is unusual (compared to Pol II) by requiring no control sequences upstream of the gene, instead normally relying on internal control sequences - sequences within the transcribed section of the gene (although upstream sequences are occasionally seen, e.g. U6 snRNA gene has an upstream TATA box as seen in Pol II Promoters).

Typical stages in 5S rRNA (also termed class I) gene initiation:

Typical stages in a tRNA (also termed class II) gene initiation:

TFIIB remains bound to DNA following initiation of transcription by Pol III (unlike bacterial σ factors and most of the basal transcription factors for Pol II transcription). This leads to a high rate of transcriptional reinitiation of Pol III-transcribed genes.

Typical stages in a U6 snRNA (also termed class III) gene initiation (documented in vertebrates only):

Polymerase III terminates transcription at small polyTs stretch (5-6). In Eukaryotes, a hairpin loop is not required, as it is in prokaryotes.

The types of RNAs transcribed from RNA polymerase III includes:

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