Abstract
The acl5 mutant of Arabidopsis thaliana shows a severe defect in stem elongation. ACL5 encodes thermospermine synthase. Thermospermine is a structural isomer of spermine. sac51-d was identified as a suppressor mutant of the dwarf phenotype of acl5 and its responsible gene SAC51 encodes a bHLH transcription factor. sac51-d contains a point mutation in the 4th of five uORFs present in the 5' leader sequence of SAC51 mRNA and increases translation of the main ORF, which may cause the recovery of stem growth. We have generated transgenic lines carrying the GUS gene fused with the SAC51 promoter and its 5' leader, in which each one of the uORFs is disrupted. Only the plants with the 4th uORF-disrupted construct showed higher levels of the GUS fusion transcript and the GUS activity than those with the wild-type construct but less increase by external therspermine than those with the other constructs tested. The SAC51 promoter-GUS construct lacking most of the SAC51 5' leader was not responsive to thermospermine. These results suggest that thermospermine acts in overcoming the 4th uORF-dependent repression of the main ORF translation and/or regulating the mRNA stability of SAC51.
