TRANSFER OF CLOVER INFECTIVITY OF RHIZOBIUM TRIFOLII TO RHIZOBIUM PHASEOLI AS MEDIATED BY AN EPISOMIC FACTOR

Abstract

The transfer of host specificity by conjugation from 1 rhizobial species to the other was studied. Rhizobium trifolii and Rh. phaseoli were mixed-cultured in yeast extract -mannitol medium and then the former was eliminated completely from the culture by the action of Rh. trifolii-specific phage. Cells which survived in the culture (strain W 1, progency of Rh. phaseoli) were examined for the clover infectivity as determined by the formation of infection threads in root hairs of clover plants. Strain W 1 clearly showed the clover infectivity, but it retained various physiological and immuno-logical characteristics of Rh. phaseoli. When Rh. trifolii and strain W 1 were cultured in the presence of acridine orange, which is known to eliminate episomic factors in bacterial cells, a marked decrease in infective activity on clover plants was demonstrated. The host specificity in rhizobia is probably mediated by an episome-like factor just as the drug resistance facor in bacteria, although evidence is lacking to prove that the factor could be integrated into the chromosome.