Abstract
Three classes of exopolysaccharide (EPS) defective mutants were isolated by Tn3Hogus-insertion mutagenesis of Rhizobium sp. strain TAL1145, which nodulates tree legumes. The class I and class III mutants produced 10-22% of the EPS produced by TAL1145 and appeared partially mucoid while the class II mutants formed small, opaque and non-mucoid colonies. Size-fractionation of the soluble EPSs made by these mutants in the culture supernatant indicated that the class I and the class III mutants produced reduced levels of both highland low-molecular-mass EPSs while the class II mutants lacked both these EPSs but produced a small amount of a medium-molecular-mass anthrone-reactive EPS. The succinyl and acetyl substituents observed in the TAL1145 EPS were absent in the EPS of the class II mutants. When examined under UV, the class I and class III mutants grown on Calcofluor-containing YEM agar showed dim blue fluorescence, compared to the bright blue fluorescence of the wild-type strain, whereas the class II mutants did not fluoresce. While the dim blue fluorescence of the class III mutants changed to yellow-green after 10 d, the fluorescence of the class I mutants did not change after prolonged incubation. Unlike the EPS-defective mutants of other rhizobia, these mutants did not show different symbiotic phenotypes on determinate- and indeterminate-nodulating tree legumes. The class I and the class III mutants formed small ineffective nodules on both types of legumes whereas the class II mutants formed normal nitrogen-fixing nodules on both types. The genes disrupted in the class I and class III mutants form a single complementation group while those disrupted in the class II mutants constitute another. All the three classes of EPS-defective mutants were located within a 10.8 kb region and complemented by two overlapping cosmids.