P-type ATPases constitute a large family of cation pumps that play crucial physiological roles in many organisms, including bacteria, plants and mammals. They are postulated to play important roles in a variety of environmental adaptation systems. Recently, we cloned two distinct putative P-type ATPase genes (pacS and pacL) from a photosynthetic cyanobacterium, Synechococcus species PCC7942. In this study, one of the gene products (named PacS) was found to possess a putative metal-binding motif (Gly-Met-X-Cys-X-X-Cys) in its N-terminal portion. Thus we supposed that this ATPase may function as a metal pump. Indeed, the results of Northern blotting analysis showed that pacS-mRNA specifically increases upon addition of copper or silver to the growth medium. The results of Western blotting analysis confirmed the view that PacS accumulates in copper-treated Synechococcus cells. Thus we concluded that the expression of PacS ATPase is regulated in response to the change in concentration of external metals, namely copper and silver. Consistent with this, an insertional inactivation mutant of pacS exhibited hypersensitivity in terms of growth to these potentially toxic metals. It was also revealed that PacS was mainly located in the thylakoid membrane, in which the photosynthetic reactions take place. This P-type ATPase in the thylakoid membrane is implicated as a copper-transporting system that may be involved in copper-homeostasis crucial to the photosynthetic thylakoid function.