A CYSG MUTANT STRAIN OF RHIZOBIUM-ETLI PLEIOTROPICALLY DEFECTIVE IN SULFATE AND NITRATE ASSIMILATION

By its inability to grow on sulfate as the sole sulfur source, a mutan t strain (CTNUX8) of Rhizobium etli carrying Tn5 was isolated and char acterized. Sequence analysis showed that Tn5 is inserted into a cysG ( siroheme synthetase)-homologous gene. By RNase protection assays, it w as established that the cysG-like gene had a basal level of expression in thiosulfate- or cysteine-grown cells, which was induced when sulfa te or methionine was used. Unlike its wild-type parent (strain CE3), t he mutant strain, CTNUX8, was also unable to grow on nitrate as the so le nitrogen source and was unable to induce a high level of nitrite re ductase. Despite its pleiotropic phenotype, strain CTNUX8 was able to induce pink, effective (N-2-fixing) nodules on the roots of Phaseolus vulgaris plants. However, mixed inoculation experiments showed that st rain CTNUX8 is significantly different from the wild type in its abili ty to nodulate. Our data support the notion that sulfate (or sulfite) is the sulfur source of R. etli in the rhizosphere, while cysteine, me thionine, or glutathione is supplied by the root cells to bacteria gro wing inside the plant.