Abstract
Loss-of-function mutants of the ACAULIS5 (ACL5) gene have a severe defect in stem elongation. ACL5 encodes a thermospermine synthase. The dominant sac51-d mutant that rescues the dwarf phenotype of acl5 disrupts the 4th uORF of SAC51, which encodes a bHLH transcription factor. SAC51 mRNA contains five uORFs in its 5' leader sequence. To address the role of these uORFs in translational control of the main ORF, we generated transgenic plants that contain the GUS reporter gene fused to the SAC51 promoter-5' leader fragment, in which each uORF is disrupted. Our results suggested that only the 4th uORF has a repressive role in the translational control of the downstream main ORF while other uORFs facilitate translation of the main ORF.SAC51 expression is up-regulated by thermospermine. To identify the cis-regulatory elements, analyses of the SAC51 promoter are currently in progress.