Involved in the biosynthesis of L-aspartate-beta-semialdehyde, which is an intermediate in the biosynthesis of ectoine, a highly soluble organic osmolyte, called compatible solute. Ectoine is used to avoid excessive water efflux, plasmolysis, molecular crowding of the cytoplasm, and cessation of growth in high salinity environments. Catalyzes the phosphorylation of the beta-carboxyl group of L-aspartate to yield 4-phospho-L-aspartate. ATP + L-aspartate = 4-phospho-L-aspartate + ADP Allosterically and strongly feedback inhibited by tryptophan. The presence of either 650 mM NaCl or KCl reduces the inhibition by tryptophan. Amine and polyamine biosynthesis; ectoine biosynthesis. Monomer. Induced in response to osmotic stress. Belongs to the aspartokinase family. Truncated N-terminus.