This superfamily represents a ssDNA-binding transcriptional regulator domain consisting of a helix-swapped dimer of beta(4)-alpha motifs. This domain is found as a C-terminal domain in the transcriptional co-activator PC4 (also known as P15; where it is a dimer of two separate motifs), and in the plant transcriptional regulator PBF-2 (where it is a single chain domain formed by a tandem repeat of two motifs).Transcriptional regulators play a critical role in controlling the level of transcription from specific genes in response to different stimuli. Members of this family of transcriptional regulators, which preferentially bind single-stranded DNA, include PBF-2 from plants, the mammalian nuclear factor 1-X (NF1-X), and positive cofactor 4 (PC4). These proteins are structurally similar, consisting of a helix-swapped dimer of beta(4)-alpha motifs.The plant defence transcription factor PBF-2 is comprised of four p24 subunits that interact through a helix-loop-helix motif to produce a central pore []. PBF-2 functions as part of the plant's defence system in response to the detection of a pathogen. Upon stimulation, PBF-2 induces several signal transduction pathways leading to changes in the expression of defence genes, including the pathogenesis-related (PR) genes.NF1-X is one of several NF1 proteins that function as transcription factors. NF1-X consists of two functionally distinct domains: a conserved N-terminal DNA-binding domain and a C-terminal transcriptional regulatory domain. NF1-X binds to the promoter for the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase gene [].PC4 (or P15) possess the ability to co-activate and suppress transcription via its DNA-binding activity. PC4 has been shown to stimulate transcription in vitrowith diverse activators, including VP16, thyroid hormone receptor, BRCA-1, often involving TFIIA. PC4 and TFIIA are thought to facilitate the assembly of the pre-initiation complex. The repressive activity of PC4 can be alleviated by the transcription factor TFIIH, which protects promoters from PC4 repression []. PC4 consists of two domains: an N-terminal regulatory domain and a C-terminal cryptic DNA-binding domain. The protein acts as a dimer with two ssDNA binding channels running in opposite directions to each other [].
