Galactinol is involved in sequence-conserved upstream open reading frame-mediated repression of Arabidopsis HsfB1 translation

Previously we reported that translation of Arabidopsis HsfB1 main open-reading frame (ORF) is repressed by its sequence-conserved upstream ORF (sc-uORF). Here we show that galactinol is involved in sc-uORF-mediated repression of AtHsfB1 translation. Among the candidate chemicals which accumulate in during heat shock, galactinol, but neither raffinose nor stachyose, was found to repress AtHsfB1 translation. Galactose, a biosynthetic precursor of galactinol, showed a similar activity but only at 1000 higher doses, while other sugars such as sucrose, glucose or fructose did not show such activity. According to its specificity and the required effective dose, we conclude galactinol is an endogenous effector to repress AtHsfB1 translation. Requirement of sc-uORF for galactinol action was confirmed by transferring to a heterologous gene on which galactinol was then active in translational regulation. sc-uORF acted in cis but not intrans configuration. The peptide sequence derived from sc-uORF (in particular the conserved carboxy-half peptide sequence) but not the nucleotide sequence was required for galactinol action. Based on our finding, we discuss a physiological significance of this metabolite-mediated negative regulation of AtHsfB1 translation during heat shock response.