Selection of several classes of mimosine-degradation-defective Tn3Hogus-insertion mutants of Rhizobium sp. strain TAL1145 on the basis of mimosine-inducible GUS activity.

Rhizobium sp. strain TAL1145 that nodulates Leucaena leucocephala degrades mimosine, a toxin produced by this tree legume. A cosmid clone, pUHR263, containing approximately 25 kb cloned DNA was isolated by plating Escherichia coli cells containing the cosmid clone library of TAL1145 on a minimal medium in which 3-hydroxy-4-pyridone (HP), a degradation product of mimosine, was used as the source of nitrogen. Cosmid pUHR263 was mutagenized by random insertions of Tn3Hogus, a transposon that makes transcriptional gus fusions when it is inserted in a gene in the correct orientation. Various pUHR263::Tn3Hogus derivatives that showed mimosine-inducible or mimosine-repressible GUS activities when transferred to the Rhizobium sp. strain TAL1145 were selected. Mutants of TAL1145 were constructed by transferring these Tn3Hogus insertions into the TAL1145 chromosome through double-homologous recombination. These mutants were classified into five classes on the basis of defects in mimosine degradation. The growth of these mutants was inhibited to different extents by mimosine applied to the growth medium. Mimosine forms a red-colored Fe-mimosine complex when FeCI3 is added to the medium. The inhibitory effect of Fe-mimosine on growth of the mutants was much less than that of mimosine.