A PROTEIN-BINDING AT-RICH SEQUENCE IN THE SOYBEAN LEGHEMOGLOBIN C3 PROMOTER IS A GENERAL CIS-ELEMENT THAT REQUIRES PROXIMAL DNA ELEMENTS TOSTIMULATE TRANSCRIPTION

A nodule nuclear factor, NAT2, interacts with two AT-rich binding site s (NAT2 BS1 and NAT2 BS2) in the soybean leghemoglobin (lb) c3 promote r. In transgenic Lotus corniculatus nodules, an oligonucleotide contai ning NAT2 BS1 activated an inactive -159 lbc3 promoter when placed imm ediately upstream of the promoter. The activation was independent of t he orientation of NAT2 BS1 but was dependent on its position in the pr omoter. The abilities of different mutated binding sites to activate e xpression in vivo were correlated to their respective in vitro affinit ies for binding NAT2. This suggested that the interaction between NAT2 and NATS BS1 is responsible for the observed reactivation. Further ac tivation experiments with the lbc3 and the leaf-specific Nicotiana plu mbaginifolia ribulose bisphosphate carboxylase/oxygenase small subunit (rbcS-8B) promoter suggested that another specific cis element(s) is required for the function of NAT2 BS1. Thus, the -102 lbc3 promoter la cking the organ-specific element (-139 to -102) was not reactivated by the presence of the binding site, and the rbcS-8B promoter required s equences between -312 and -257 to be activated by NAT2 BS1. This impli es that NAT2 has to work in combination with other trans-acting factor (s) to increase expression. The finding of NAT2-like binding activitie s in different plant organs and the specific expression of the hybrid NAT2 BS1/-312 rbcS-8B promoter in leaves suggest that NAT2 is a genera l activator of transcription.