Copper-induced expression, cloning, and regulatory studies of the plastocyanin gene from the cyanobacterium Synechocystis sp. PCC 6803

Abstract

Plastocyanin can be detected in Synechocystis sp. PCC 6803 when 3 μM copper is added to the growth medium, BG-11. The plastocyanin gene ( petE ) was cloned from a genomic λ EMBL 3 library by screening with the petE gene from Anabaena sp. PCC 7937. The Synechocystis 6803 petE gene is present as a single copy and, as deduced from the DNA sequence, encodes a precursor protein of 126 amino acids. The predicted 29 amino acid transit peptide shows substantial homology to the Anabaena 7937 transit peptide, thought to direct the plastocyanin precursor to the thylakoid lumen. Putative promoter sites −16 and −38 base pairs from the start of the petE gene have been identified. The deduced amino acid sequence has the greatest homology (61%) to the green alga Scenedemus obliquus plastocyanin. Despite the lower homology, the copper binding residues and certain aromatic residues remain highly conserved. Northern hybridization analysis indicates that the Synechocystis sp. PCC 6803 petE gene is not transcriptionally regulated since the accumulation of petE mRNA appears to be independent of the copper concentration in the growth media. The possibility of an additional polypeptide needed to facilitate the electron transfer from plastocyanin to P700 + is also discussed.