Role of N gamma-acetyldiaminobutyrate as an enzyme stabilizer and an intermediate in the biosynthesis of hydroxyectoine

Strain CHR63 is a salt-sensitive mutant of the moderately halophilic wild-t ype strain Halomonas elongata DSM 3043 that is affected in the ectoine synt hase gene (ectC). This strain accumulates large amounts of N gamma-acetyldi aminobutyrate (NADA), the precursor of ectoine (D. Canovas, C. Vargas, F. I glesias-Guerra, L. N. Csonka, D. Rhodes, A. Ventosa, and J. J. Nieto, J. Bi ol. Chem. 272:25794-23801, 1997). Hydroxyectoine, ectoine, and glucosylglyc erate were also identified by nuclear magnetic resonance (NMR) as cytoplasm ic organic solutes in this mutant. Accumulation of NADA, hydroxyectoine, an d ectoine was osmoregulated, whereas the levels of glucosylglycerate decrea sed at higher salinities. The effect of the growth stage on the accumulatio n of solutes was also investigated. NADA was purified from strain CHR63 and was shown to protect the thermolabile enzyme rabbit muscle lactate dehydro genase against thermal inactivation. The stabilizing effect of NADA was gre ater than the stabilizing effect of ectoine or potassium diaminobutyrate. A H-1 NMR analysis of the solutes accumulated by the wild-type strain and mu tants CHR62 (ectA::Tn1732) and CHR63 (ectC::Tn1732) indicated that H. elong ata can synthesize hydroxyectoine by two different pathways-directly from e ctoine or via an alternative pathway that converts NADA into hydroxyectoine without the involvement of ectoine.