Underexpression of ap from R-plasmids in fast-growing Rhizobium species.

The presence of the plasmid RP1 in the cells of Rhizobium leguminosarum strains Rld1, 300, and 248, R. phaseoli 1233, R. trifolii strains T1 and 6661, and R. meliloti 4013 was found to appreciably increase bacterial resistance toward kanamycin and tetracycline but not toward ampicillin. The presence of 16 other R-plasmids in R. leguminosarum was also found to either not increase or only marginally increase bacterial resistance toward ampicillin. It appears now that underexpression of the plasmid-specified ampicillin function is common to most fast- and slow-growing rhizobia.