The genetic basis of symbiosis, mucoid structure and melanin biosynthesis i
n a local isolate of Rhizobium cicer was investigated. The strain was avery
effective symbiont and produced substantial amounts of exopolysaccharides
and melanin. It harbored three high-molar-mass plasmids of 55, 80 and 130 k
b, respectively. Thirty-one melanin production-negative (Mep(-)) derivative
s were obtained through plasmid curing. The plasmid patterns of cured deriv
atives indicated that the loss of 55 and/or 80 kb plasmids was sufficient f
or obtaining Mep- phenotype. The specific involvement of 55-kb plasmid in m
elanin production was confirmed upon re-introduction of this plasmid into o
ne of the Mep- derivatives. Further investigation also indicated that the 1
30-kb plasmid might be necessary for both mucoidal appearance and symbiotic
functions.