Abstract
Institute of Applied Microbiology, University of Tokyo, Bunkyo-ku, Tokyo (Received October 26, 1967) The transfer of host specificity by conjugation from one 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 remained survived in the culture
(strain W1, progeny of Rh. phaseoli) was examined for the clover infectivity as determined by the formation of infection threads in root
hairs of clover plants. Strain W1 clearly showed the clover infectivity, but it retained various physiological and immunological characteristics
of Rh. phaseoli. When Rh. trifolii and strain W1 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 present observations strongly support the view that the host specificity in rhizobia is mediated by an episome-like factor just like as the drug resistance factor in bacteria, although the evidence is lacked in that the factor could be integrated into the chromosome.
