Catalyzes the circularization of gamma-N-acetyl-alpha,gamma-diaminobutyric acid (ADABA) to ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidine carboxylic acid), which is an excellent osmoprotectant. Does not act on N-acetylated amino acids like N-alpha-acetyl-L-asparagine,N-alpha-acetyl-L-ornithine, N-alpha-acetyl-L-lysine and N-epsilon-acetyl-L-lysine. (2S)-4-acetamido-2-aminobutanoate = H2O + L-ectoine Seems to require potassium ions for its activity and stability. Slightly inhibited by N-ethylmaleimide. Optimum pH is 8.5-9.0. Optimum temperature is 0-10 degrees Celsius in the presence of 0.05 M NaCl, 15 degrees Celsius in the presence of 0.77 M NaCl, and 30 degrees Celsius in the presence of 3.0 M NaCl. Amine and polyamine biosynthesis; ectoine biosynthesis; L-ectoine from L-aspartate 4-semialdehyde: step 3/3. Belongs to the ectoine synthase family.