Transcriptional coactivator that stimulates transcriptional activity by bridging regulatory proteins and TBP, thereby recruiting TBP to promoters occupied by DNA-binding regulators. Involved in the tolerance to heat and osmotic stress by partially activating the ethylene-response signal transduction pathway. Binds to TPS5. PubMed:16283071 shows a localization restricted to the nucleus and not altered by heat or ABA treatment, while PubMed:18201973 describes a transport from the cytoplasm to the nucleus induced by heat stress. A number of isoforms are produced. According to EST sequences. Expressed in leaves, roots, stems, flowers, siliques and shoots. Not detected in seeds. Scarcely expressed in early stages, restricted to the shoot apical meristem in 14-day-old seedlings and detected in all tissues in 28-day-old seedlings. By abscisic acid, H(2)O(2), heat treatment, dehydration and high salt, but not by cold treatment, 2,4-D, ACC, methyl jasmonate or salicylic acid. Plants are deficient in basal thermotolerance. Belongs to the MBF1 family.