RESUMEN
Twenty one cysteine and 13 methionine auxotrophs of Sinorhizobium meliloti Rmd201 were obtained by random mutagenesis with transposon Tn5. The cysteine auxotrophs were sulfite reductase mutants and each of these auxotrophs had a mutation in cysI/cysJ gene. The methionine auxotrophs were metA/metZ, metE and metF mutants. One hundred per cent co-transfer of Tn5-induced kanamycin resistance and auxotrophy from each Tn5-induced auxotrophic mutant indicated that each mutant cell most likely had a single Tn5 insertion. However, the presence of more than one Tn5 insertions in the auxotrophs used in our study cannot be ruled out. All cysteine and methionine auxotrophs induced nodules on alfalfa plants. The nodules induced by cysteine auxotrophs were fully effective like those of the parental strain-induced nodules, whereas the nodules induced by methionine auxotrophs were completely ineffective. The supplementation of methionine to the plant nutrient medium completely restored symbiotic effectiveness to the methionine auxotrophs. These results indicated that the alfalfa host provides cysteine but not methionine to rhizobia during symbiosis. Histological studies showed that the defective symbiosis of methionine auxotrophs with alfalfa plants was due to reduced number of infected nodule cells and incomplete transformation of bacteroids.