A guaB mutant strain of Rhizobium tropici CIAT899 pleiotropically defective in thermal tolerance and symbiosis

Rhizobium tropici strain CIAT899 displays a high intrinsic thermal toleranc e, and had been used in this work to study the molecular basis of bacterial responses to high temperature. We generated a collection of R. tropici CIA T899 mutants affected in thermal tolerance using Tn5-luxAB mutagenesis and described the characterization of a mutant strain, CIAT899-10T, that fails to grow under conditions of high temperature. Strain CIAT899-10T carries a single transposon insertion in a gene showing a high degree of similarity w ith the guaB gene of Escherichia coli and other organisms, encoding the enz yme inosine monophosphate dehydrogenase, The guaB strain CIAT899-10T does n ot require guanine for growth due to an alternative pathway via xanthine de hydrogenase and, phenotypically, in addition to the thermal sensitivity, th e mutant is also defective in symbiosis with beans, forming nodules that la ck rhizobial content. Guanine and its precursors restore wild-type toleranc e to grow at high temperature. Our data show that, in R. tropici, the produ ction of guanine via inosine monophosphate dehydrogenase is essential for g rowth at extreme temperatures and for effective nodulation.