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J. Bacteriol., 12 1997, 7343-7350, Vol 179, No. 23
R Tate, A Riccio, M Iaccarino and EJ Patriarca
By its inability to grow on sulfate as the sole sulfur source, a mutant
strain (CTNUX8) of Rhizobium etli carrying Tn5 was isolated and
characterized. Sequence analysis showed that Tn5 is inserted into a cysG
(siroheme synthetase)-homologous gene. By RNase protection assays, it was
established that the cysG-like gene had a basal level of expression in
thiosulfate- or cysteine-grown cells, which was induced when sulfate or
methionine was used. Unlike its wild-type parent (strain CE3), the mutant
strain, CTNUX8, was also unable to grow on nitrate as the sole nitrogen
source and was unable to induce a high level of nitrite reductase. Despite
its pleiotropic phenotype, strain CTNUX8 was able to induce pink, effective
(N2-fixing) nodules on the roots of Phaseolus vulgaris plants. However,
mixed inoculation experiments showed that strain CTNUX8 is significantly
different from the wild type in its ability to nodulate. Our data support
the notion that sulfate (or sulfite) is the sulfur source of R. etli in the
rhizosphere, while cysteine, methionine, or glutathione is supplied by the
root cells to bacteria growing inside the plant.
Copyright © 1997, American Society for Microbiology
A cysG mutant strain of Rhizobium etli pleiotropically defective in sulfate and nitrate assimilation
International Institute of Genetics and Biophysics, CNR, Naples, Italy.
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