The Roles of Two TATA Boxes and 3′-flanking Region of Soybean β-Tubulin Gene (tubB1) in Light-sensitive Expression

The soybean tubB1 gene is expressed primarily in the germinating seedling and is strongly down regulated in response to light in the upper hypocotyl. Previous studies demonstrate that the 1 kb 5′-flanking region of this gene is sufficient for its appropriate expression in etiolated seedlings. Transient expression studies demonstrated that the presence of the tubBI 3′-flanking sequence element decreased reporter gene expression as compared to the nopaline synthase (NOS) 3′-flanking sequence element. In this study we investigated the ability of the 3′flanking region to influence the expression of a β-glucuronidase (GUS) reporter gene in transgenic tobacco and Arabidopsis. The presence of the tubB1 3′-flanking sequence element in chimera constructs reduced reporter gene expression specifically in the hypocotyl and petioles of light-grown, transgenic seedlings. Additionally, site-directed muta-genesis of the two TATA sequences in the 1 kb tubB1 5′-flanking sequence element (TATA box A in the -122 to -117 bp region and TATA box B in the -35 to -30 region) showed that both elements are functional and additive in controlling tubB1 gene expression in seedling tissues. While transcription from TATA box A was predominant regardless of lighting conditions, the relative usage of TATA box B increased in the dark. We conclude that both TATA box sequences are utilized to direct expression of the tubB1 gene to the cotyledons, hypocotyl and root tip of germinating seedlings that are regions of cell expansion and that the 3′-flanking sequence element down-regulates expression in the hypocotyl in response to light. Thus, it is plausible that the tubB1 protein may play an important role in cell expansion in seedling development requiring its regulated expression by light.