Abstract
The nodulation ability was effectively eliminated from different Rhizobium trifolii strains incubated at elevated temperature (Żurkowski and Lorkiewicz, 1978). Non-nodulating (Nod-) mutants were stable and no reversion of Nod- to Nod+ phenotype was observed. Strains R. trifolii 24 and T12 which showed a high percentage of elimination of nodulation ability were examined in detail. Two plasmids were detected in strain 24 using neutral and alkaline sucrose gradient centrifugation of plasmid preparations. Molecular weights of the plasmids pWZ1 and pWZ2 were 460 Mdal and 190 Mdal, respectively. Rhizobium lysates labeled with 3H-thymidine and ultracentrifuged in caesium chloride — ethidium bromide gradients demonstrated a 40% reduction of the plasmid DNA content in R. trifolii 24 Nod- mutants in comparison with the nodulating wild type strain 24. It was found further that non-nodulation of mutants 24 Nod- was due to the absence of plasmid pWZ2. Sucrose gradient data also demonstrated that strain T12 contained two plasmids with molecular weights corresponding to those of pWZ1 and pWZ2, respectively. In Nod- mutant clones derived from strain T12, pWZ2 plasmid was missing.
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