A tobacco cryptic constitutive promoter, tCUP, revealed by T-DNA tagging

We have isolated a constitutive promoter sequence, tCUP, from tobacco by T-DNA tagging using a promoterless GUS-nos3' reporter gene construct. The T-DNA integration event produced a translational fusion with the GUS gene that is expressed widely in organs, at both the mRNA and enzyme activity levels. In tobacco transformed with a tCUP-GUS-nos3' gene, GUS specific activity in leaves was within a range of values similar to those of plants transformed with the widely used constitutive promoter gene fusion, CaMV 35S promoter-GUS-nos3'. Characteristics of the tCUP promoter sequence differ from those of other plant constitutive promoters; for instance, the tCUP sequence lacks a TATA box. Transcription initiates at a single site within the tCUP sequence which is similar to a transcriptional start site consensus sequence determined for plant genes. The tCUP promoter is cryptic as RNA accumulation at the transcriptional start site is not detected in untransformed tobacco. Thus, tCUP is the first example of a cryptic, constitutive promoter isolated from plants. The tCUP-GUS-nos3' gene fusion produced GUS activity in tissues of all species tested suggesting that tCUP may utilize fundamental transcription mechanisms found in plants.