Analysis of a sunflower polyubiquitin promoter by transient expression

We have recently isolated two sunflower polyubiquitin genes (UbB1 and UbB2) which are induced by heat-stress and show a highly conserved region of about 250 bp upstream of the transcription initiation site. The promoter activity of this region has been analyzed by transient expression using the β-glucoronidase (GUS) gene as reporter in tobacco protoplasts. The GUS expression in absence of any external inducer is five times higher than that observed under the control of the CaMV 35S promoter. Analysis of different deletions within the 250-bp region showed that a 65-bp sequence is responsible for most of the promoter activity in tobacco protoplasts. This activity is intron-independent and is not enhanced when protoplasts are heat-stressed. Sequence homologies strongly suggest that UbB1 plays a major role in the stress response in sunflower.