TFIIIA

TFIIIA has nine zinc fingers, but other proteins have anything from one to more than 60 zinc finger motifs within their sequence. 

III in vitro and in cultured Xenopus kidney cells [60]. Further studies used polyamides in combination with recombinant derivatives of TFIIIA subunits to elucidate essential minor groove contacts for the binding of this TF (Tab. 3.1) [62]. 

Figure 39-14. Zinc fingers are a series of repeated domains (two to nine) in which each is centered on a tetrahedral coordination with zinc. In the case of TFIIIA, the coordination is provided by a pair of cysteine residues (C) separated by 12-13 amino acids from a pair of histidine (H) residues. In other zinc finger proteins, the second pair also consists of C residues. Zinc fingers bind in the major groove, with adjacent fingers making contact with 5 bp along the same face of the helix.

Figure 2.20 Schematic representation of a zinc-finger domain from TFIIIA and related proteins. X represents any amino acid conserved amino acids are histidine (H), cysteine (C), tyrosine (Y), phenylalanine (F), and leucine (L).

Both biochemical and X-ray crystallographic data show that binding of TFIIIA to the 5S rRNA gene internal control region utilizes all but the fourth and sixth of the nine zinc fingers of the transcription factor (Figure 12.15). 

ZnP C2H2 (smart00355) YRCPECGKVEKSKSA LQEHMRTH First identified in the Xenopus transcription factor TFIIIA. Found in numerous nucleic add-binding proteins. Ragl (NP 000439), Strin/ Rfpl38 (NP 057355) 380... 

The first zinc binding motif discovered was that of the eucaryotic transcription factor TFIIIA of Xenopus laevis which contains 9 copies of a Cys2His2-Zinc motif The structure of the binding motif is shown in Fig. 1.4. The central zinc ion serves to pack an a-hehx against a P-sheet and thereby position the a-helix. The recognition of the DNA sequence occurs via this a-helix. 

Small GC clusters 5 bp apart Binding site for TFIIIA... 

After TFIIIA binds, proteins TFIIIC and then TFII-IB also bind. Although promoters of classes 2 and 3 do not require TFIIIA, all three classes depend upon TFIIIB and TFIIIC.484 The TATA-binding protein TBF is one of three components present in TFIIIB, which may be regarded as the true initiation factor.47 485 Both TFIIIA and TFIIIC can be described as assembly factors 47 A silkworm RNA pol III has been reported to require a transcription factor consisting of RNA 486... 

PolIII polymerase also transcribes tRNA genes. In this case TFIIIA does not participate in the transcription process (see fig. 28.12c). 

Eukaryotic cells contain multiple copies of the 5S rRNA gene. Unlike other eukaryotic rRNA genes, the 5S rRNA genes are transcribed by RNA Pol III. Two control elements, an A box and a C box, lie downstream of the transcriptional start site. The C box binds TFIIIA which then recruits TFIIIC. TFIIIB now binds and interacts with RNA Pol III to form the transcription initiation complex. Transcription produces a mature 5S rRNA that requires no processing. 

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