Seven slow-growing strains of cowpea rhizobia ( sp.) were tested for ability to receive the P plasmids pRD1 and R68.45 from The frequency of transfer was 2 x 10--2 x 10- for five strains including CB756, but no transfer was detected for the other two. The his-nif segment of pRD1 was lost at high frequency in CB756 transconjugants whilst pRD1 and R68.45 and antibiotic resistance genes were stable through 50-100 generations in non-selective media even though growth rates of CB756 were almost halved in transconjugant clones. Furthermore, plasmids were maintained in three strains tested during passage through root nodules, although there were some differences in symbiotic expression between transconjugants and their wild-type parents. A colour change was effected by CB756 in the presence of 6-cyanopurine and therefore could possibly be used as an indicator of expression in addition to acetylene reduction in culture.


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