Aspartate beta-D-semialdehyde dehydrogenase genes (asd) were cloned and sequenced from a deep-sea-adapted strictly barophilic bacterium, Shewanella sp. strain DB6705, and a moderately barophilic bacterium, Shewanella sp. strain DSS12. The determined asd sequences of these two strains were very similar, and the identity of the deduced amino acids sequences was 96.2%. The 5'-ends of the asd mRNA from both strains were localized at corresponding sites by primer extension analysis, and two transcriptional starting points, which differed by only 1 base, were detected. In strain DB6705, a pressure-regulated transcript was mainly observed, whereas in strain DSS12, a pressure-tolerant transcript was observed together with the pressure-regulated transcript. Western-blotting analysis showed that the ASD protein was expressed under higher pressure conditions in DB6705, and under all pressure conditions tested in DSS12, as reflected in the primer extension results. Our findings suggest that asd expression controlled by pressure is one of the important mechanisms involved in the adaptation of microorganisms to the deep-sea environment.