A cyanobacterium, Synechococcus species PCC7942, has a gene encoding a copper-transporting P-type ATPase, which is located in the thylakoid membrane. At the 5'-upstream of this ATPase gene, we identified another gene, which was supposed to be implicated in a copper-transport process. This novel gene was found to encode a putative pore-forming membrane protein that belongs to a growing family of homologous intrinsic membrane proteins (the MIP family of proteins), which include the major intrinsic protein (MIP) from animal lens fibre junction membranes, the tonoplast intrinsic protein (TIP) from vacuolar membranes of higher plants, and the Escherichia coli glycerol facilitator (GlpF) in the cytoplasmic membrane. The deduced product, named SmpX (Synechococcus membrane protein), is highly homologous throughout its entire sequence to these intrinsic membrane proteins which were postulated to be pore-forming proteins involved in a variety of transport processes. The primary amino acid sequence of SmpX shares all properties characteristic for members of the MIP family. SmpX is more similar to the eukaryotic members (e.g., nodulin-26 from soybean) than to the prokaryotic ones.