Summary: One-step mutants of sp. strain 32h1 resistant to d-cycloserine showed substantial (≥ 90%) loss of asymbiotic (agar culture) nitrogenase activity but only partial (25 to 50%) loss of symbiotic (root nodule) nitrogenase activity. Two- and three-step resistant mutants showed a further decline or complete loss of both forms of nitrogenase activity. Since -cycloserine inhibited the uptake of alanine by the parent strain and since the mutants possessed a defective transport system for alanine, it is suggested that the mutants are unable to concentrate -cycloserine. Agar cultures of a one-step resistant mutant contained the three morphological types of Rhizobium present in the parent culture. In contrast, only one morphological type of Rhizobium was found in cultures of two- and three-step resistant mutants. Root nodules formed by a one-step resistant mutant on cowpea contained fewer bacteroid-filled plant cells than were present in nodules formed by the parent strain. Root nodules formed by two- and three-step resistant mutants contained only a small number of infected plant cells and in many of these rhizobia had not been released from infection threads. It is suggested that changes in the permeability of the Rhizobium cell wall, resulting in resistance to -cycloserine, are responsible for the morphological changes and loss of nitrogenase activity in these mutants.


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