A promoter sequence involved in cell-specific expression of the pea glutamine synthetase GS3A gene in organs of transgenic tobacco and alfalfa.

The DNA sequence of the pea cytosolic glutamine synthetase GS3A gene promoter has been determined and the start of transcription mapped using S1 nuclease. The full-length promoter and a series of 5' deletions were fused to beta-glucuronidase (GUS) and introduced into transgenic tobacco and alfalfa. In transgenic tobacco the GS3A promoter directed GUS expression in the phloem cells of the vasculature in leaves, stems and roots. GUS expression was also detected in the vasculature of cotyledons and the root tips of germinating T1 seedlings. The promoter conferred a similar expression pattern in transgenic alfalfa, and expression was also observed in root nodules. Nodule expression was located in nodule primordia, as well as the meristem, symbiotic zone, and vasculature of mature nodules. The promoter was found to be active even when deleted to -132 relative to the start of transcription. DNA mobility-shift analysis identified a protein present in nuclear and whole-cell plant extracts which bound to a 17 bp DNA element contained within the minimal -132 promoter required for expression.