RNA
Polymerase III Transcription
RNA polymerase III (pol III) is the largest and most complex of the eukaryotic RNA polymerases. It involves 14 subunits, totaling 700,000 daltons. All of the genes it transcribes are small, they are not all translated into proteins, and their transcription is regulated by certain sequences that lie within the transcribed region. The major targets for pol III are the genes for all the tRNAs and for the 5S ribosomal RNA. Like the major ribosomal RNA genes, these small genes are present in multiple copies, but they are usually not grouped together in tandem arrays, nor are they localized in one region of the nucleus. Rather, they are scattered over the genome and throughout the nucleus.
5S 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.
Transcription factors - The interaction of the three transcription factors, the polymerase, and the gene is shown in Figure 28.21. TFIIIA makes contact with DNA over a length of about 40 bp. This somehow makes the gene accessible to TFIIIB, TFIIIC, and polymerase III. TFIIIA can also complex with 5S RNA. This propensity limits 5S RNA production when the product is in excess. TFIIIA is a DNA-binding protein, in which metal-binding zinc "fingers" make contact with and identify DNA sequences (Figure 28.22). The fingers fit into the major grooves of the DNA, as do the helices in helix--turn--helix and leucine zipper proteins (Figure 28.23; see also here). TFIIIA is a monomer. TFIIIB is dimeric. TFIIIC is an enormous complex, involving six polypeptide chains and covering the whole 5S rRNA or tRNA gene. Once pol III is attached, it can produce multiple transcripts before dissociating.
INTERNET LINKS: