Bacteroids Isolated from Ineffective Nodules of Pisum sativum Mutant E135 (syml3) Lack Nitrogenase Activity but Contain the Two Protein Components of Nitrogenase

Pea mutant E135 (sym15) forms ineffective (Fix−) nodules that lack nitrogen fixing activity. To determine the developmental step blocked in E135 nodules we studied the nitrogenase activities in isolated bacteroids and in cell-free extracts of bacteroids, and measured the two components of nitrogenase protein in bacteroids. Bacteroids prepared anaerobically from E135 nodules showed no acetylene reduction activity in the presence and absence of myoglobin. Furthermore, no acetylene reduction activity by cell-free extracts of E135 bacteroids was detected in the presence of ATP-generating system and dithionite. However, immuno-blotting analyses revealed the presence of nitrogenase components I and II in E135 nodule bacteroids. These results suggest that a host plant gene is involved in the expression of nitrogenase activity in symbiotic bacteria.