*3.5. BrHSP18.2s Promoters and Their Possible Control*

The induction of *Arabidopsis HSP18.2* (*Hsp18.1-CI*/AT5G59720) expression by HSFA2, a major thermotolerance HSF [24,26,31], is related to the role of HSFA2 in sustaining H3K4 methylation [24]. The promoter activity of *ArabidopsisHSP18.2* is highest at 35 ◦C [55]. This promoter contains eight HSE modules between −97 and −53 bp; the deletion of two modules maintains promoter activity, but a deletion of six modules causes a dramatic reduction in promoter activity [26]. All *BrHSP18.2* promoters contain seven HSE modules, implying that there is no difference in BrHSFA2 binding among the three *BrHSP18.2* promoters. In other organisms, at least two nGAAn units arranged head-to-head (nGAAnnTTCn) or tail-to-tail (nTTCnnGAAn) are required in the promoters of *HSPs* for efficient HSF binding [25]. Four HSE units were found in all *BrHSP18.2s* promoters, one of which is overlapping (Figure S4), indicating that *BrHSP18.2A* to *C* contain sufficient numbers of HSEs for TF binding. The binding of HSFA2 with several target genes including *HSP18.2* has been assessed [26], showing that two modules of a TATA-proximal HSE (nGAAnnTTCn) are essential for transcriptional activation by HSFA2. These modules are also conserved in all *BrHSP18.2s* promoters, suggesting that the expression differences among *BrHSP18.2s* upon HT exposure or warming conditions might be due to the presence of different numbers of HSFs such as *BrHSFA2* (which is controlled by alternative splicing) and/or other factor(s).
