Characterization of a 14 kDa component with low expression in a unique Nod⁺ Fix^- Bradyrhizobium japonicum

A pronase-sensitive, 14 kDa component of bacteroids of Bradyrhizobium japonicum I-110ARS was identified and characterized using monoclonal antibodies. This component was weakly synthesized or was missing in bacteroids of a unique Nod⁺ Fix^- mutant, ARS2525. Both the 14 kDa component and poly-β-hydroxybutyrate (PHB) were located in the same protein peak after sucrose density gradient separation of lysed bacteroids. Thirty-five-day-old bacteroids of B. japonicum I-110ARS contained up to 10 times more PHB than B. japonicum ARS2525 bacteroids. Immunocytochemistry of ultra-thin nodule sections showed that the component was associated with PHB granules in the bacteroids, possibly with the limiting membrane around the granules. This observation implies that the component may be similar to other storage lipid body proteins. Whether low synthesis of the 14 kDa component or low PHB content was a cause, a consequence or related to the Fix^- phenotype was not established.