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. (2S)-4-acetamido-2-aminobutanoate = H2O + L-ectoine Amine and polyamine biosynthesis; ectoine biosynthesis; L-ectoine from L-aspartate 4-semialdehyde: step 3/3. Belongs to the ectoine synthase family.