ANALYSES OF PHOSPHOENOLPYRUVATE CARBOXYLASE GENE STRUCTURE AND EXPRESSION IN ALFALFA NODULES

Phosphoenolpyruvate carboxylase (PEPC) plays a crucial role in the ass imilation of CO2 during symbiotic N-2 fixation in legume root nodules. In this study, an alfalfa PEPC gene (PEPC-7), whose transcripts are f ound at elevated revels in nodules relative to either leaves or roots, has been isolated and characterized. The intron/exon structure of thi s gene is identical to that of most other plant PEPC genes except for the presence of an additional intron in the 5' untranslated region. In situ RNA hybridization studies showed that PEPC transcripts were pres ent in the nodule meristem, the infection zone, the nitrogen-fixing zo ne, and the parenchyma. PEPC transcripts were also found in vascular t issue of roots and nodules and in the pulvinus of petioles. In transge nic alfalfa, a chimeric reporter gene was expressed in these same regi ons except that little expression was found in the nodule meristem. An alyses of promoter deletions suggest that the region between -634 and -536 is of particular importance in directing transcriptional activity to the infected zone of nodules. Within this region is a mirror repea t sequence that is potentially capable of forming an H-DNA structure. These results indicate that PEPC-7 has a central role in nitrogenfixin g nodules and that regulation of transcription is an important determi nant of its activity.