Intracellular site of synthesis and localization of leghemoglobin in root nodules

Abstract

A majority of the total protein synthesis in host cell cytoplasm is inhibited by cycloheximide. Because leghemoglobin represents a large proportion of total cellular protein in the nodule, this observation suggests that leghemoglobin may be translated on the 80S-type ribosomes. Analysis of the nascent peptides isolated from free and membrane-bound polysomes showed that free polysomes contain more immunoreactive material against antibodies to leghemoglobin as compared to that of membrane-bound polysomes. When free and membrane-bound polysomes were incubated in a wheat embryo cell-free protein-synthesizing system, a larger percentage of the released polypeptides from free polysomes was found to be immunoreactive. Similar results were obtained by translation in vitro of the poly(A)-containing mRNA isolated from free and membrane-bound polysomes. For immunocytochemical localization of leghemoglobin, the antibodies were conjugated with ferritin. Antibody conjugates were strictly localized in the host cell cytoplasm and adjacent to the outer surface of the membrane surrounding the bacteroids. Ferritin was not found on the inner surface of the membrane or within the membrane sac. These data suggest that leghemoglobin is synthesized preferentially on the free polysomes in the host cell cytoplasm, directed by a poly(A)-containing 9S mRNA that is most likely of plant origin, and that its location is restricted to the host cell cytoplasm.