Multiprotein Bridging Factor 1 (MBF1) was first purified from silkworm silk glands as a non- DNA binding transcription cofactor (Li et al., 1994). It belongs to Helix-Turn-Helix superfamily of proteins consisting of N-terminal domain which interacts with transcription factors and so called as ‘Multiprotein Bridging Factor 1, N-terminal’ and conserved TATA box binding protein (TBP) binding C-terminal helix-turn-helix domain (Aravind et al., 2005, Jaimes-Miranda and Chávez Montes., 2020). MBF1 enhances transcription activation by bridging a basic region/leucine zipper (bZIP) type transcription factor and TBP in yeast (Takemaru et al., 1998), Drosophila (Liu et al., 2003) and human (Kabe et al., 1999). Arabidopsis genome encodes three MBF1 family coactivators, MBF1a, MBF1b and MBF1c classified into group I consisting of  MBF1a, MBF1b  due to similar gene structure and MBF1c belonging to group II, and these MBFs showed tissue specific expression profiles indicating different functions (Tsuda et al., 2004, Tsuda and Yamazaki., 2004).

Arabidopsis, MBF1a gene encodes 16 kDa protein which is 82.4% identical to maize Salt Tolerance 41 (SAT41) which plays a role in response to elevated salt tolerance. AtMBF1a expression was induced in response to elevated salt and glucose levels. Also overexpression of MBF1a increased resistance to fungal disease and insensitivity to glucose. Thus implying MBF1a role in biotic and abiotic stress tolerance in Arabidopsis (Kim et al., 2007). Arabidopsis MBF1c is a key regulator of heat tolerance. In response to heat it is localized to the nucleus and functions upstream of SA, trehalose, ethylene and PR related protein 1. MBF1c interacts with heat inducible TPS5 (Trehalose Phosphate Synthase 5) component of trehalose signaling pathway and TPS5 mutants are thermosensitive. However, MBF1c does not play a role in expression of any HSFs (Suzuki et al., 2008). Transgenic Arabidopsis overexpressing AtMBF1 fused to an active transcription repression domain (SRDX) resulted in suppression of leaves cell expansion during late leaf development stage along with lower ploidy levels in the leaves, also elevated expression of negative regulators of endoreduplication were observed. Thus suggesting AtMBF1s role in cell expansion and ploidy regulation during leaf development stage (Tojo et al., 2008). Arabidopsis MBF1s triple knock down mutant abc-  showed increased sensitivity to hydrogen peroxide and methyl viologen oxidative stress and sorbitol mediated osmotic stress. Overexpression of AtMBF1c in abc- triple mutant resulted in partial or complete phenotype rescue based on physiological or development conditions. ABR1, a member of the AP2/ERF family acting as a ABA repressor, is regulated by AtMBF1s. Thus, AtMBF1s functions as a crosstalk regulatory component for ethylene, ABA and stress signal pathways (Arce et al., 2009). In response to heat stress, based on molecular dynamic simulations AtMBF1c N-terminal region comprising of Asparagine 67, Threonine 68, Lysine 69 and Lysine 70 residues specifically binds to CTAGA binding element in the minor groove of the DNA to form stable DNA-MBF1 complex (Jaimes-Miranda et al., 2020).

Overexpression of wheat, Triticum aestivum L. MBF1c in yeast and rice resulted in increased thermotolerance. Also, since TaMBF1c promoter region contained three heat shock elements (HSEs) and Trehalose phosphate synthase gene, their transcript levels were higher in TaMBF1c transgenic lines as compared to wild type in response to heat stress (Qin et al., 2014). Wild relative of cultivated barley, Hordeum brevisubulatum genome encodes three MBF1 proteins- HbMBF1a, HbMBF1band HbMBF1c. HbMBF1a was identified from transcriptomic data of H. brevisubulatum exposed to salt stress. Overexpression of HbMBF1a in Arabidopsis thaliana, resulted in enhanced salt tolerance and ABA insensitivity along with significant upregulation of stress related genes (Zhang et al., 2020).

Last updated June 2023 by Ankita Abnave

References:

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