The dct operon is required for the transport and utilization of the C4-dicarboxylates succinate and malate. Symbiotic strains of Mesorhizobium sp. contain two copies of the dct operon, one on the symbiosis island and one elsewhere on the chromosome. Non-symbiotic mesorhizobia contain the chromosomal operon but most are unable to utilize succinate. In addition, M. loti strain R7A symbiosis island dctB and dctD mutants are Dct" suggesting that the chromosomal dct operon is not normally expressed. Non-symbiont strains CJ1 and N18 were found to mutate to Dct+ after extended periods on succinate media, suggesting that these strains undergo some form of activating mutation. These observations indicate that the chromosomal operon is cryptic. The aims of this work were to determine why the chromosomal operon is cryptic and to identify the adaptive mutation occurring in CJ1.

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