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Salinivibrio sp. DL6 is a moderately halophilic bacterium that can grow over a wide range of salinity, 0.5-20% Nad. In response to the external osmotic stress (1-3 M NaCl), strain DI6 can accumulate ectoine, glycine betaine and glutamate as compatible solutes. Transposon mutagenesis appears to be a potentially useful tool in the research on the synthesis of compatible solutes in response to the high osmotic stress. We used suicide plasmids pSUPl 01 to introduce the transposon Tnl732 into Salinivibrio sp. DI6 via Escherichia coli SM10 mediated conjugation. One Tnl732-induced mutant, MU1, which was severely affected in salt tolerance, was isolated. Mutant MU1 could not grow above 1.5 M NaCl and did not synthesize ectoine but accumulated ectoine precursor, N-gamma-acetyl-diaminobutyrate, which was shown by 1H-NMR analysis. From these data, we concluded that in the strain Salinivibrio sp. DI6 ectoine performs a key role for the adaptation high salinity environments, since glycine betaine and glutamate were accumulated regardless of the salinity.