Transcript levels of the polyubiquitin gene McUBI1 had been reported to be constant during Crassulacean acid metabolism (CAM) induction in the facultative CAM plant, Mesembryanthemum crystallinum. Here, we report the sequences of the full-length cDNA of McUBI1 and its promoter, and validation of the McUBI1 promoter as an internal control driving constitutive expression in transient assays using the dual-luciferase system to investigate the regulation of CAM-related gene expression. The McUBI1 promoter drove strong, constitutive expression during CAM induction. We compared the activities of this promoter with those of the cauliflower mosaic virus (CaMV) 35S promoter in detached C3- and CAM-performing M. crystallinum and tobacco leaves. We confirmed stable expression of the genes controlled by the McUBI1 promoter with far less variability than under the CaMV 35S promoter in M. crystallinum, whereas both promoters worked well in tobacco. We found the McUBI1 promoter more suitable than the CaMV 35S promoter as an internal control for transient expression assays in M. crystallinum.

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