TFIIA interacts with the TBP subunit of TFIID and aids in the binding of TBP to TATA-box containing promoter DNA. Interaction of TFIIA with TBP facilitates formation of and stabilizes the preinitiation complex. Interaction of TFIIA with TBP also results in the exclusion of negative (repressive) factors that might otherwise bind to TBP and interfere with PIC formation. TFIIA also acts as a coactivator for some transcriptional activators, assisting with their ability to increase, or activate, transcription. The requirement for TFIIA in vitro transcription systems has been variable, and it can be considered either as a GTF and/or a loosely associated TAF-like coactivator. Genetic analysis in yeast has shown that TFIIA is essential for viability.
TFIIA is encoded by two separate genes, one of which encodes a large subunit (TFIIAalpha/beta, TFIIAL, TOA1; gene name GTF2A1) and another which encodes a small subunit (TFIIAgamma, TFIIAS, TOA2; gene name GTF2A2). In humans, the sizes of the encoded proteins are approximately 55 kD and 12 kD. Both genes are present in species ranging from humans to yeast, and their protein products interact to form a complex composed of a beta barreldomain and an alpha helical bundle domain. It is the N-terminal and C-terminal regions of the large subunit that participate in interactions with the small subunit. These regions are separated by another domain whose sequence is always present in large subunits from various species but whose size varies and whose sequence is poorly conserved. The large subunit is often observed to be proteolytically processed into two smaller subunits (alpha and beta) of approximately 35 kD and 19 kD. A second gene encoding a large TFIIA subunit has been found in some higher eukaryotes. This gene, ALF/TFIIAtau (gene name GTF2A1LF) is expressed only in oocytes and spermatocytes, suggesting it has a TFIIA-like regulatory role for gene expression only in germ cells.
^DeJong J, Roeder RG (1993). "A single cDNA, hTFIIA/alpha, encodes both the p35 and p19 subunits of human TFIIA". Genes Dev.7 (11): 2220–34. doi:10.1101/gad.7.11.2220. PMID8224848.
^Ozer J, Moore PA, Bolden AH, Lee A, Rosen CA, Lieberman PM (1994). "Molecular cloning of the small (gamma) subunit of human TFIIA reveals functions critical for activated transcription". Genes Dev.8 (19): 2324–35. doi:10.1101/gad.8.19.2324. PMID7958899.